Electrolyzer for gaseous carbon dioxide

ABSTRACT

An electrochemical device and method can include techniques involving bipolar membrane electrolysis to transform an input product into an output product. Some embodiments can include a gas-diffusion electrode as a cathode, a bipolar membrane configured to facilitate autodissociation, and an anode that can be configured as a liquid-electrolyte style electrode or a gas-diffusion electrode. In some embodiments the electrochemical device can be configured as a CO 2  electrolyzer that is designed to utilize input product including carbon dioxide gas and water to generate output products that can include gaseous carbon monoxide or other reduction products of carbon dioxide and gaseous oxygen or the oxidation products of a depolarizer such as hydrogen, methane, or methanol. Embodiments can be utilized in the production of fuels or feedstocks for fuels and carbon-containing chemicals, in air purification systems, flue gas treatment devices, and other machines and facilities.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is related to and claims the benefit of priorityof U.S. Provisional Patent Application Ser. No. 62/577,357 filed on Oct.26, 2017, the entire contents of which is incorporated herein byreference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT

This invention was made with government support under Grant No.DE-FG02-07ER15911 awarded by the Department of Energy. The Governmenthas certain rights in the invention.

FIELD OF THE INVENTION

Embodiments can relate to an electrochemical device capable of gas phaseelectrolysis and bipolar membrane electrolysis.

BACKGROUND OF THE INVENTION

Conventional electrochemical reduction of carbon dioxide systems andmethods can be appreciated from U.S. Pat. No. 9,481,939, U.S. Pat. No.9,181,625, U.S. Pat. No. 9,085,827, U.S. Pat. Publ. No. 2017/0183789,U.S. Pat. Publ. No. 2013/0118911, and Pat. Publ. No. CN 102912374.Conventional systems may be inefficient, have poor stability, and/orhave difficulty in separating reaction products from the electrolytes.These and other disadvantages may limit the use of conventionalelectrochemical reduction systems.

BRIEF SUMMARY OF THE INVENTION

Embodiments can be related to an electrochemical device that may includetechniques involving gas phase electrolysis and bipolar membraneelectrolysis to transform an input product into an output product. Someembodiments can include an electrochemical device having at least oneelectrochemical cell, each electrochemical cell having a cathode, amembrane, and an anode. In some embodiments, input product can beintroduced into the electrochemical device at the cathode. This caninclude introducing an input product in a gas phase. Reactions at thecathode can transform the input product into reduced chemical products.Some of these reduced chemical products can be caused to exit theelectrochemical device as output product. Some of these reduced chemicalproducts can be caused to react with the membrane to generate additionalchemical products. The additional chemical products can be caused toreact with the anode. This can generate additional output product.

As a non-limiting example, carbon dioxide gas and water may beintroduced into the electrochemical device at the cathode. Reactions atthe cathode can transform the carbon dioxide gas into reduction productsof carbon dioxide and oxygen as output products. The reduction productsof carbon dioxide and the oxygen may be directed out from theelectrochemical device. In some embodiments, water can be introducedinto the electrochemical device at the anode. Liquid electrolyte and/orthe anode can electrochemically drive the oxidation of the water tooxygen as an output product. The oxygen can then be directed out fromthe electrochemical device. In some embodiments, a depolarizer such asmethane, hydrogen, or methanol can be introduced to the anode of thecell and its oxidation products may be directed out from theelectrochemical device. Some embodiments can include a gas-diffusionanode. This may be used to generate an electrochemical device without aliquid electrolyte.

With some embodiments, introduction of input product as a gas can allowfor reaction products to be generated in the gas phase. This may alsoallow for collection of output product in the gas phase. These gas phaseproducts can eliminate the need to provide product separationtechniques, as no product is being dissolved in a liquid electrolytesolution. As no reactant is being dissolved in a liquid electrolytesolution, the reactants are not caused to transport through a liquid,which can improve upon the transport rate of chemical species within theelectrochemical device.

Some embodiments can include use of a bipolar membrane. Embodiments ofthe bipolar membrane can be used to separate the cathode and the anode,as well as isolate the reactants associated with the cathode and isolatethe reactants associated with the anode. Embodiments of the bipolarmembrane can also be configured to manage flux of chemical species fromthe bipolar membrane to the cathode and/or to the anode. For example,the bipolar membrane can be used to provide a flux of protons to thecathode and a flux of hydroxide ions to the anode. This may generate anelectrochemical device that can eliminate or reduce undesired crossoverof chemical product between the cathode and anode. This can also allowthe electrochemical device to operate with a stable electrolyte pH, evenunder long-term operation.

While various embodiments may describe an electrochemical deviceconfigured for carbon dioxide electrolysis into carbon monoxide andoxygen, other forms of output product can be generated. For example, itis contemplated for embodiments of the electrochemical device to be usedfor carbon dioxide electrolysis into syngas (carbon monoxide+hydrogen)and oxygen. Syngas can be used as a precursor to hydrocarbon fuels,other fuels, and other high value chemicals (e.g., propane, gasoline,methanol, dlmethylether (DME), formate, methane, methanol, ethyleneglycol, butanol, etc. It is also contemplated for embodiments of theelectrochemical device that different cathode catalysts may be chosen toreduce carbon dioxide directly to other carbon-containing products, suchas formic acid, acetic acid, ethylene, propylene, methanol, ethanol,propanol and ethylene glycol.

In one embodiment, an electrochemical device can include anelectrochemical cell comprising a cathode, an anode, and a membrane. Atleast a portion of the cathode can be separated from at least a portionof the anode by the membrane. The cathode can have a gas-diffusionelectrode. The anode can have at least one of a liquid-electrolyte styleelectrode and a gas-diffusion electrode. The membrane can be a bipolarmembrane. The bipolar membrane can be configured to maintain a flux ofprotons to the cathode and also maintain a flux of hydroxide ions to theanode. The electrochemical cell can be configured to receive carbondioxide gas and water and output reduction products of carbon dioxideand oxygen.

In some embodiments, the bipolar membrane can include a cation exchangemembrane and an anion exchange membrane. In some embodiments, thebipolar membrane can be configured to promote autodissociation. In someembodiments, the bipolar membrane further can have a membrane catalyst.In some embodiments, the membrane catalyst can be at least one of asilicate, an amine polymer, a graphite oxide, and an anolyte solution.In some embodiments, the anion exchange membrane can be laminated by acation-exchange polymer film. In some embodiments, the cation-exchangepolymer film can be a sulfonated tetrafluoroethylene basedfluoropolymer-copolymer. In some embodiments, the cation-exchangepolymer film can be a sulfonated poly(ether ether ketone) polymer. Insome embodiments, the cation-exchange polymer film can be a polymericweak acid, such as poly(acrylic acid). In some embodiments, thecation-exchange film can contain an inorganic cation exchanger such as aclay, a layered transition metal oxide, or graphite oxide, either aloneor as a polymer composite. In some embodiments, a surface of the cationexchange membrane can be patterned and/or a surface of the anionexchange membrane can be patterned. In some embodiments, the cathode canbe a cathode catalyst. In some embodiments, the cathode catalyst can begold, silver, copper, indium, bismuth, lead, tin, tellurium, and/orgermanium. In some embodiments, the cathode catalyst can be mixed with abinder, a polymeric electrolyte coating, and/or an ionic liquid. In someembodiments, the anode can be an anode catalyst. In some embodiments,the anode catalyst can be at least one of iridium oxide, rutheniumalloys, mixed oxides of ruthenium containing iridium and/or platinum,mixed metal oxides containing cobalt, nickel, iron, manganese,lanthanum, cerium, copper, nickel borate, cobalt phosphate, NiFeOx.

In one embodiment, an electrochemical device can include anelectrochemical cell having a cell first end and a cell second end. Theelectrochemical device can have a cathode with a gas-diffusionelectrode. The electrochemical device can have an anode with at leastone of a liquid-electrolyte style electrode and a gas-diffusionelectrode. The electrochemical device can have bipolar membraneseparating at least a portion of the cathode from at least a portion ofthe anode. The electrochemical device can have a cathode flow mediumcomprising carbon. The electrochemical device can have an anode flowmedium comprising carbon. The electrochemical device can have a frameconfigured to hold the cathode flow medium, the cathode, the bipolarmembrane, the anode, and the anode flow medium together.

In some embodiments, at least one of the cathode flow mediums and theanode flow medium has at least one of a cell inlet and a cell outlet. Insome embodiments, the frame has at least one pass-through regioncorresponding with at least one of the cell inlets and the cell outlet.In some embodiments, the frame seals the electrochemical cell except atthe at least one pass-through region. In some embodiments, the cathodehas a cathode catalyst configured as a reduction catalyst. In someembodiments, the anode has an anode catalyst configured as an oxidationcatalyst.

In one embodiment, a carbon dioxide electrolyzer can include anelectrochemical cell comprising a cathode, an anode, and a membrane. Atleast a portion of the cathode can be separated from at least a portionof the anode by the membrane. The cathode can have a gas-diffusionelectrode. The anode can have at least one of a liquid-electrolyte styleelectrode and a gas-diffusion electrode. The membrane can be a bipolarmembrane. The cathode can be a cathode catalyst configured as a carbondioxide reduction catalyst. The anode can be an anode catalystconfigured as a water oxidation catalyst.

In one embodiment, an electrochemical device can include anelectrochemical cell having a cell first end and a cell second end. Theelectrochemical device can have a cathode comprising a gas-diffusionelectrode. The electrochemical device can have an anode comprising atleast one of a liquid-electrolyte style electrode and a gas-diffusionelectrode. The electrochemical device can have a bipolar membraneseparating at least a portion of the cathode from at least a portion ofthe anode. The electrochemical device can have a cathode flow mediumcomprising carbon. In some embodiments, the cathode flow medium can belocated between the cell first end and the cathode. In some embodiments,at least one cell inlet can be formed in the cathode flow mediumconfigured to receive carbon dioxide gas. In some embodiments, at leastone cell outlet can be formed in the cathode flow medium configured tooutput carbon monoxide gas and/or water. In some embodiments, the devicecan have an anode flow medium comprising carbon. The anode flow mediumcan be located between the cell second end and the anode. At least onecell inlet can be formed in the anode flow medium configured to receivewater and/or electrolyte. At least one cell outlet can be formed in theanode flow medium configured to output oxygen. The bipolar membrane canbe configured to maintain a flux of protons to the cathode and a flux ofhydroxide ions to the anode.

In one embodiment, a method of reducing product crossover in anelectrochemical cell can involve configuring a bipolar membrane of anelectrochemical to cause ions to travel towards an anode electrode and acathode electrode of the electrochemical cell when the electrochemicalcell is under an applied current condition.

In some embodiments, the method can involve the bipolar membrane beingconfigured to supply protons (H⁺) to the cathode and to cause water(H₂O) to self-ionize via autodissociation to generate hydroxide ions(OH⁻). In some embodiments, the method can involve the bipolar membranebeing configured to supply the OH⁻ to the anode. In some embodiments,the method can involve generating a reverse bias to provide a flux of H⁺to the cathode. In some embodiments, the method can involve the flux ofH⁺ opposing the direction of product crossover in the electrochemicalcell. In some embodiments, the method can involve configuring thebipolar membrane to have an anion exchange layer and a cation exchangelayer joined together at an interfacial layer, the interfacial layerconfigured to catalyze the autodissociation of H₂O. In some embodiments,the method can involve depositing at least one catalyst layer on theinterfacial layer. In some embodiments, the method can involve tuningthe water dissociation reaction at the interfacial layer via adjusting atype of the catalyst and/or an amount of the catalyst.

In one embodiment, a bipolar membrane can include a cation exchangelayer and an anion exchange layer, the cation exchange layer beingadjacent the anion exchange layer to form a cation-anion exchangejunction region. The bipolar membrane can include at least one catalystlayer formed within the cation-anion exchange junction region. In someembodiments, at least one catalyst layer can be configured to decreasethe electric field intensity applied across the cation-anion exchangejunction region. In some embodiments, at least one catalyst layer isgraphite oxide.

In at least one embodiment, a bipolar membrane can include a cationexchange layer and an anion exchange layer, the cation exchange layerbeing adjacent the anion exchange layer to form a cation-anion exchangejunction region. In some embodiments, the cation-anion exchange junctioncan be configured so that the cation exchange layer interpenetrates theanion exchange layer and/or the anion exchange layer interpenetrates thecation exchange layer. In some embodiments, the interpenetrating cationexchange layer and anion exchange layer can generate a plurality oftransport pathways for water dissociation products H⁺ and OH⁻ to flow.

A method of reducing product crossover in an electrochemical cell of anelectrochemical device can include various steps. These steps caninclude, for example, configuring a bipolar membrane of anelectrochemical cell that is positioned between an anode and a cathodeto cause ions to travel towards an anode electrode and a cathodeelectrode of the electrochemical cell when the electrochemical cell isunder an applied current condition and operating the electrochemicalcell so that the bipolar membrane facilitates a supply of protons (H⁺)to the cathode, to cause water (H₂O) to self-ionize via autodissociationto generate hydroxide ions (OH⁻) and protons H⁺ to supply a flux of theOH⁻ to the anode and supply a flux of the H⁺ to the cathode.

In some embodiments of the method of reducing product crossover in theelectrochemical cell of an electrochemical device, the electrochemicaldevice can be a carbon dioxide electrolyzer, the cathode can include acathode catalyst configured as a carbon dioxide reduction catalyst, theanode can include an anode catalyst configured as a water oxidationcatalyst, and the electrochemical cell can include: (i) a cathode flowmedium between the cathode and the bipolar membrane and there is atleast one cell inlet of the cathode flow medium configured to receivecarbon dioxide and at least one cell outlet of the cathode flow mediumconfigured to output carbon monoxide gas and/or water; and (ii) an anodeflow medium between the anode and the bipolar membrane, at least onecell inlet of the anode flow medium is configured to receive waterand/or an electrolyte, and at least one cell outlet of the anode flowmedium is configured to output oxygen. For such embodiments, theoperating of the electrochemical cell can include: feeding water and/oran electrolyte to the anode flow medium; feeding a flow of carbondioxide to the cathode flow medium; outputting gaseous oxygen from theanode flow medium; and outputting carbon monoxide and/or water from thecathode flow medium.

Further features, aspects, objects, advantages, and possibleapplications of the present invention will become apparent from a studyof the exemplary embodiments and examples described below, incombination with the Figures, and the appended claims.

BRIEF DESCRIPTION OF THE FIGURES

The above and other objects, aspects, features, advantages and possibleapplications of the present invention will be more apparent from thefollowing more particular description thereof, presented in conjunctionwith the following drawings, in which:

FIG. 1 shows a first exemplary embodiment of an electrochemical device.

FIG. 2 shows an exploded view of the first exemplary embodiment of anelectrochemical device.

FIG. 3 shows an embodiment of an electrochemical cell that may be usedin the first exemplary embodiment of an electrochemical device.

FIG. 4 shows a cut-away view of the first exemplary embodiment of anelectrochemical device.

FIG. 5 is another view of the first exemplary embodiment of anelectrochemical device.

FIG. 6 is a graph shows stability data for an embodiment of anelectrochemical device.

FIG. 7 is a graph comparing cell potential over time for an embodimentof the electrochemical device and a conventional electrochemical deviceusing a nafion cation exchange membrane.

FIG. 8 is a graph showing a current-voltage curve for an embodiment ofthe electrochemical device operating at high current density.

FIGS. 9 and 10 each shows a faradaic efficiency plot for a conventionaldevice having a bipolar membrane electrolyzer with an aqueousbicarbonate catholyte. These graphs demonstrate examples of degradationof electrode selectivity that often occurs in conventional devices.

FIG. 11 shows a current density plot of a conventional bipolar membranecompared to an embodiment of a bipolar membrane that may be used with anembodiment of the electrochemical device.

FIG. 12 is an exemplary block diagram showing the transport of methanolby electroosmosis through a conventional anion-exchange membrane.

FIG. 13 is an exemplary block diagram showing outward flux of H⁺ and OH⁻that can occur in an embodiment of a bipolar membrane.

FIG. 14 is graphs showing crossover of formate, methanol, and ethanolversus time in exemplary electrochemical cells having an anion exchangemembrane and an embodiment of the bipolar membrane.

FIG. 15 shows graphs illustrating crossover of formate and methanol atdifferent applied currents after 2 hours in exemplary electrochemicalcells having an anion exchange membrane and an embodiment of the bipolarmembrane.

FIG. 16 shows graphs illustrating crossover of formate and methanol atzero current density with 0.5 M KHCO₃ used as electrolyte on both thecathode and anode sides of exemplary electrochemical cells having ananion exchange membrane and an embodiment of the bipolar membrane.

FIG. 17 shows a schematic of the preparation of an embodiment of thebipolar membrane having an exemplary interfacial catalyst layer and ascanning electron microscope (SEM) image of the bipolar membrane.

FIG. 18 is a plot showing J-E curves of embodiments of the bipolarmembrane having an exemplary interfacial catalyst layer prepared by anexemplary layer-by-layer technique.

FIG. 19 is a plot showing the water dissociation rate constant kdmeasured for embodiments of the bipolar membrane having an exemplaryinterfacial catalyst layer.

FIG. 20 is a plot showing water dissociation reaction resistance Rwmeasured for embodiments of the bipolar membrane having an exemplaryinterfacial catalyst layer.

FIG. 21 is a plot showing depletion region thickness as a function ofreverse bias voltage for embodiments of the bipolar membrane having anexemplary interfacial catalyst layer.

FIG. 22 is a graph showing J-E curves for embodiments of the bipolarmembrane having an exemplary interfacial catalyst layer.

FIG. 23 is a graph showing potential distribution profiles forembodiments of the bipolar membrane having an exemplary interfacialcatalyst layer.

FIG. 24 is a graph showing concentration profiles of the waterdissociation products H⁺ and OH⁻ for embodiments of the bipolar membranehaving an exemplary interfacial catalyst layer.

FIG. 25 is a graph showing electrolyte KNO₃ ion distributions forembodiments of the bipolar membrane having an exemplary interfacialcatalyst layer.

FIG. 26 shows schematic drawings of the depletion region for embodimentsof the bipolar membrane having an exemplary interfacial catalyst layerand embodiments without an exemplary interfacial catalyst layer, alongwith enlarged views of the cation-anion exchange junction for each. Thethickness of the black arrows indicate the higher electric field in thebipolar membrane without the exemplary interfacial catalyst layer.

FIG. 27 shows a graph of electric field intensity at a cation-anionexchange junction for embodiments of the bipolar membrane having anexemplary interfacial catalyst layer and embodiments without anexemplary interfacial catalyst layer.

FIG. 28 shows a graph of electric field intensity at cation-anionexchange junction for embodiments of the bipolar membrane having anexemplary interfacial catalyst layer and embodiments without anexemplary interfacial catalyst layer.

FIG. 29 shows a scanning electron microscope image and a schematic of acation-anion exchange junction of an embodiment of a 3D bipolar membranewith intertwined anion exchange layer-cation exchange layer fibers.

FIG. 30 is a graph showing J-E curves for a cation-anion exchangejunction of an embodiment of a 3D bipolar membrane.

FIG. 31 is a graph showing the water dissociation rate constant kd for acation-anion exchange junction of an embodiment of a 3D bipolarmembrane.

FIG. 32 is a graph showing the water dissociation reaction resistance Rwfor a cation-anion exchange junction of an embodiment of a 3D bipolarmembrane.

FIG. 33 is a graph showing depletion region thickness d as a function ofreverse bias voltage for a cation-anion exchange junction of anembodiment of a 3D bipolar membrane.

FIG. 34 is a graph showing J-E curves for embodiment of a bipolarmembrane.

FIG. 35 is a graph showing steady-state performance of embodiment of abipolar membrane.

DETAILED DESCRIPTION OF THE INVENTION

The following description is of an embodiment presently contemplated forcarrying out the present invention. This description is not to be takenin a limiting sense, but is made merely for the purpose of describingthe general principles and features of the present invention. The scopeof the present invention should be determined with reference to theclaims.

Referring to FIGS. 1-4, various embodiments of the apparatus disclosedherein can include an electrochemical device 10 capable of gas phaseelectrolysis and bipolar membrane electrolysis. Embodiments of theelectrochemical device 10 can be configured to generate an outputproduct from an input product. The input product can be a gas, a liquid,a solid or combinations thereof e.g. a slurry, gas having solidparticulates entrained therein, a liquid having solid particlesentrained therein, etc.). The output product can be a gas, a liquid, asolid, or a combination thereof (e.g. a slurry, a gas having solidparticulates entrained therein, etc.). In some embodiments, both theinput product and the output product include a gas. In some embodiments,the output product can be a reduced chemical product of the inputproduct, an oxidized product of the input product, and/or a combinationof both.

Some embodiments of the electrochemical device 10 can be configured asan electrolyzer. For example, embodiments of the electrochemical device10 can be configured to use electric current to drive chemical reactionsthat may facilitate generating the output product from the inputproduct. In some embodiments, the electrochemical device 10 can beconfigured as a carbon dioxide (CO₂) electrolyzer. As a non-limitingexample, the electrochemical device 10 can be configured to receivecarbon dioxide (CO₂) gas as an input product at the cathode 14.Reactions within the electrochemical device 10 can generate carbonmonoxide (CO), water (H₂O), and/or hydrogen (H₂) as an output product.The CO, the H₂O, and/or the H₂ may be caused to exit the electrochemicaldevice 10 for capture or further processing. In some embodiments, theH₂O can be caused to self-ionize at the membrane 22 via autodissociationto generate protons (H⁺) and hydroxide ions (OH⁻). Some of the H₂Ogenerated at the cathode 14 can be caused to move to the anode 18.Additional H₂O can be introduced into the electrochemical device 10 asinput product at the anode 18. The additional H₂O can be in the form ofa liquid or a vapor. If the additional H₂O is in the form of a liquid,the OH⁻ may be used to react with the anode 18 via electrolyte of theelectrochemical device 10 to generate oxygen (O₂) and/or H₂O asadditional output product. If the additional H₂O is in the form of avapor, the OH⁻ may be used to react directly with the anode 18 ofelectrochemical device 10 to generate oxygen (O₂) and/or H₂O asadditional output product. The O₂ and/or the H₂O may be caused to exitthe electrochemical device 10 for capture or further processing.

As explained herein, other input products can be used, such ashumidified CO₂ gas, for example. The input product can also be a mixtureof gases that may include gases other than CO₂ gas. In addition, otheroutput products can be generated, such as formic acid, formate, methane,methanol, ethylene, ethylene glycol, butanol, etc. For example, in thesituation in which the device 10 is used for CO₂ reduction to CO, thedevice 10 can be configured to receive CO₂ gas as an input and generateCO₂ and O₂. In the situation in which the device 10 is used for CO₂reduction to products other than CO (e.g., formate, methanol, ethylene,etc.), the device 10 can be configured to receive CO₂ gas and H₂O asinputs and generate reduction products of and CO₂ and O₂. The reductionproducts of CO₂ can include but are not limited to formic acid,methanol, methane, formaldehyde, acetaldehyde, acetic acid, glyoxal,ethanol, ethene, ethane, ethylene glycol, dimethyl ether, methylformate, propene, propane, n-propanol, isopropanol, isomers of butanol,as well as mixtures of these products and hydrogen.

Some embodiments of the electrochemical device 10 can include anelectrochemical cell 12 structure. Embodiments of the electrochemicalcell 12 structure can include a cathode 14 within a cathode flow medium16 and an anode 18 within an anode flow medium 20. The cathode flowmedium 16 and/or cathode 14 can be separated from the anode flow medium20 and/or anode 18 by a membrane 22. The electrochemical cell 12 can beconfigured to facilitate intake of an input product. The input productcan enter the electrochemical cell 12 at a cell inlet 26. Theelectrochemical cell 12 can be configured to generate an output productfrom the input product. The electrochemical cell 12 can be configured totransform the input product at the cathode 14 via a reduction reaction.The electrochemical cell 12 can be configured to transform the inputproduct at the anode 18 via an oxidation reaction. The output productcan exit the electrochemical cell 12 at a cell outlet 28.

The electrochemical cell 12 may include a cathode 14. The cathode 14 maybe positioned adjacent or within the cathode flow medium 16. The cathodeflow medium 16 can be positioned at a cell first end 30 of theelectrochemical cell 12. The electrochemical cell 12 can include ananode 18. The anode 18 may be positioned adjacent or within the anodeflow medium 20. The anode flow medium 20 can be positioned at a cellsecond end 32 of the electrochemical cell 12. The electrochemical cell12 can include a membrane 22. The membrane 22 may be disposed betweenthe cell first end 30 and the cell second end 32. This can include beingdisposed between the cathode 14 and the anode 18. A volume of spacebetween the cathode 14 and the cell first end 30 of the electrochemicalcell 12 can be referred to as a cathode flow medium 16. The cathode flowmedium 16 can include a carbon material filled within the volume ofspace between the cathode 14 and the cell first end 30 of theelectrochemical cell. A volume of space between the anode 18 and thecell second end 32 can be referred to as an anode flow medium 20. Theanode flow medium 20 can include a carbon material or a graphite oxidematerial filled within the volume of space between the anode 18 and thecell second end 32. In some embodiments, the membrane 22 can separate atleast a portion of the cathode 14 from at least a portion of the anode18. This can include a physical separation, a chemical separation (e.g.,chemical isolation), an electrical separation (e.g., electricalisolation), etc.

The electrochemical cell 12 can include any number of cathodes 14,anodes 18, and/or membranes 22. For example, the electrochemical cell 12can include a single cathode 14 or a plurality of cathodes 14. With aplurality of cathodes 14, each cathode 14 may be stacked against eachother in a serial formation, in a staggered formation, or in any othertype of formation. The electrochemical cell 12 can include a singleanode 18 or a plurality of anodes 18. With a plurality of anodes 18,each anode 18 may be stacked against each other in a serial formation,in a staggered formation, or in any other type of formation. Themembrane 22 can include a single membrane 22 or a plurality of membranes22. With a plurality of membranes 22, each membrane 22 may be stackedagainst each other in a serial formation, in a staggered formation, orin any other type of formation. The electrochemical cells 12 may also bestacked in series to create a multi-cell electrolyzer.

The cathode flow medium 16 can be configured as a flow compartment. Thiscan include allowing flow of input product and/or output product. Thecathode flow medium 16 can include a cell inlet 26 to allow forintroduction of input product. The cathode flow medium 16 can include acell outlet 28 to allow for removal of output product. The anode flowmedium 20 can be configured as a flow compartment. This can includeallowing flow of input product, electrolyte, and/or output product. Theanode flow medium 20 can include a cell inlet 26 to allow forintroduction of input product and/or electrolyte. The anode flow medium20 can include a cell outlet 28 to allow for removal of electrolyteand/or output product.

Any one or both of the cathode flow medium 16 and/or anode flow medium20 can include a desired shape or path. For example, a portion of thecathode flow medium 16 can have a pathway 24 formed into a portionthereof or onto a surface thereof. A portion the anode flow medium 20can have a pathway 24 formed into a portion thereof or onto a surfacethereof. The pathway 24 can facilitate flow of fluid (e.g., inputproduct, output product, electrolyte, etc.) through the electrochemicalcell 12. In some embodiments, the pathway 24 can be configured tomaximize the amount of contact the fluid has with an electrode of theelectrochemical cell 12. For example, the cathode flow medium 16 caninclude a pathway 24 at an interface between the cathode flow medium 16and the cathode 14. The pathway 24 can direct flow of the input productand/or output product to maximize the amount to contact (e.g., surfacearea, time, etc.) the input product and/or output product has with asurface of the cathode 16 while the input product and/or output productis within the electrochemical cell 12. The anode flow medium 20 caninclude a pathway 24 at an interface between the anode flow medium 20and the anode 18. The pathway 24 can direct flow of the input product,output product, and/or electrolyte to maximize the amount to contact(e.g., surface area, time, etc.) the input product, output product,and/or electrolyte has with a surface of the anode 18 while the inputproduct, the output product, and/or electrolyte is within theelectrochemical cell 12. In addition, or in the alternative, the pathway24 can be configured to minimize the amount of contact or provideanother predetermined amount of contact of fluid with an electrode ofthe system.

The pathway 24 can be straight, serpentine, zigzagged, spiraled, etc.The shape and size of any pathway 24 can be the same as or differentfrom another pathway 24. The number, shape, dimension, and size of anypathway 24 of the cathode flow medium 16 can be the same as or differentfrom the number, shape, dimension, and size of any pathway 24 of theanode flow medium 20. The shape, size, dimension, and path direction canbe used to influence kinetics, fluid dynamics, etc. In some embodiments,any of the pathways 24 can be in fluid communication with any one of thecell inlets 26 and/or cell outlets 28.

The cathode 14 can include an electrical contact 34 configured totransport electrical charge. The anode 18 can include an electricalcontact 34 configured to transport electrical charge. In someembodiments, the electrical contact 34 of the cathode 14 and theelectrical contact 34 of the anode 18 can be placed into electricalconnection with a load 36 for transmission of electrical current.

The electrochemical device 10 can be operated in a galvanostatic mode,in which the anode 18 can be maintained at a constant current. Theelectrochemical device 10 can be operated in a potentiostatic mode, inwhich the potential difference between the cathode 14 and the anode 18can be held constant. For example, the product selectivity can exhibit avoltage dependence behavior (e.g., at different voltages, the ratio ofproducts is different). This can be used as a control parameter sincethe required ratio of hydrogen and carbon monoxide (for a CO₂ gas input)is different for different subsequent reactions. Thus, one can controlthe ratio of the products by simply controlling the voltage of thereaction in potentiostatic mode. This may be suitable for applicationswhere a dynamic response is required. For galvanostatic mode, a constantflow rate of the products can be generated. This may be more suitablefor a stationary system (e.g., where one single desired mix of productsmay be required for larger scale operation).

In some embodiments, the electrochemical device 10 can include a frame38. The frame 38 can be a structure that holds the electrochemical cell12 together and/or seals the electrochemical cell 12. Sealing caninclude forming a fluid (e.g., gas and/or liquid) seal so as to preventany fluid from entering and/or exiting the electrochemical cell 12except at a selected pass-through region 40. For example, the frame 38can be structured so that is generates a fluid seal around theelectrochemical cell 12, but includes a non-sealed portion to allowfluid to pass there-through. The non-sealed portion can be thepass-through region 40. The pass-through region 40 can be an opening inthe frame 38, a permeable portion of the frame 38, a semi-permeableportion of the frame 38, etc. The frame 38 can be made from metal,polymer, rubber, etc. The frame 38 can also have any of a number ofdifferent shapes and sizes (e.g. cubical in shape, disc in shape,polygonal in shape, elliptical in shape, etc.) to meet a particular setof design criteria. The frame 38 can be configured so that theelectrochemical cell 12 can be incorporated into a machine, a facility,or other type of device (e.g. conduit of an electricity generationplant, conduit of an exhaust conduit for an engine, incorporated into agas turbine arrangement, incorporation into a flue gas treatmentapparatus, incorporation into a heating, ventilation, and airconditioning (HVAC) system of a building, inclusion into an airpurification system of a vehicle, etc.).

In at least one embodiment, the electrochemical device 10 can include anelectrochemical cell 12 having a frame 38 that holds the electrochemicalcell 12 together. For example, the frame 38 can be a structure thatholds the cathode 14, the membrane 22, and the anode 18 of theelectrochemical cell 12 in a serial configuration. The electrochemicaldevice 10 can have a plurality of sides. For example, theelectrochemical device 10 may have a cubic structure with a device firstside 42 a, a device second side 42 b, a device third side 42 c, a devicefourth side 42 d, a device fifth side 42 e, and a device sixth side 42 fThe electrochemical cell 12 can be configured such that the cell firstend 30 is adjacent the device first side 42 a. The cell second end 32can be adjacent the device second side 42 b. The device third side 42 ccan be the top. The device fourth side 42 d can be the bottom. Thedevice fifth side 42 e can be the front. The device sixth side 42 f canbe the rear. While the various embodiment describe and illustrate thedevice 10 as having a cubic structure, other shapes and number of sidescan be used to form the device 10.

The frame 38 can form a seal around the electrochemical device 10 exceptfor at a pass-through region 40. For example, a first pass-throughregion 40 can be formed into the frame 38 to facilitate introduction ofinput product into the electrochemical cell 12. This can includefacilitating introduction of input product to a cell inlet 26 of thecathode flow medium 16. A second pass-through region 40 can be formedinto the frame 38 to facilitate removal of output product from theelectrochemical cell 12. This can include facilitating removal of outputproduct from a cell outlet 28 of the cathode flow medium 16.

A third pass-through region 40 can be formed into the frame 38 tofacilitate introduction of electrolyte into the electrochemical cell 12.This can include facilitating introduction of electrolyte to a cellinlet 26 of the anode flow medium 20. A fourth pass-through region 40can be formed into the frame 38 to facilitate removal of output productfrom the electrochemical cell 12. This can include facilitating removalof output product from a cell outlet 28 of the anode flow medium 20.Some embodiments can include introduction of input product into a cellinlet of the anode flow medium 20. A fifth pass-through region 40 can beformed into the frame 38 to facilitate introduction of input productinto a cell inlet 26 of the anode flow medium 20. Some embodiments caninclude removal of electrolyte for processing and re-introduction backinto the electrochemical cell 12. This can facilitate recycling of theelectrolyte. The electrolyte can be removed through the thirdpass-through region 40. Alternatively, a sixth pass-through region 40can be formed into the frame 38 to facilitate removal of electrolytefrom a cell outlet 28 of the anode flow medium 20.

More or fewer cell inlets 26, cell outlets 28, and/or pass-throughregions 40 can be used. The portion(s) of the frame 38 that do generatea seal can prevent and/or inhibit introduction or removal of inputproduct, output product, electrolyte, and/or other fluids. The flowrates for the fluid passed into the cell inlets 26, out of the celloutlets 28, or conveyed via the pass-through regions 40 can be affectedor driven by one or more flow control mechanisms in fluid communicationwith the electrochemical cell 12. Such flow control mechanisms caninclude valves in addition to pumps or fans. Other devices (e.g. acompressor or a combustor) that are in fluid communication with theelectrochemical cell can also be controlled to affect the flow rate ofthe fluid passed into and out of the electrochemical cell 12. Forexample, the electrolyte may be fed via an electrolyte source that is influid communication with an electrolyte cell inlet 26 and the inputproduct can be fed into the cell via at least one input product cellinlet 26 that is in fluid communication with at least one source for theinput product (e.g. an engine, a combustor, etc.). The output productcan exit the electrochemical cell via at least one cell outlet 28. Thepass-through regions 40 may be one or more defined conduits within theframe 38 of the electrochemical cell 12 that facilitate the flow offluid within the cell. There may be packing material within the conduitsor other elements therein as well to help facilitate a desired flowrate, a desired residence time, provide a catalytic effect, or otheroperational parameter of the electrochemical cell.

Fluids (e.g., input product, output product, and/or electrolyte) can beintroduced into the electrochemical cell 12 and/or removed from theelectrochemical cell 12 via a pump (peristaltic pump, rotary pump,impulse pump, etc.). The pump can be configured to force fluid into theelectrochemical cell 12 or draw fluid from the electrochemical cell 12.Any number or combination of pumps can be in fluid communication with aynumber or combination of pass-through regions 40.

Some embodiments of the frame 38 can provide a single pass-throughregion 40 in any device side or a plurality of pass-through regions 40in any device side. The shape and size of any pass-through region 40 canbe the same as or different from another pass-through region 40. Thenumber, shape, dimension, and size of any pass-through region 40 on onedevice side can be the same as or different from the number, shape,dimension, and size of any pass-through region 40 on another deviceside. The number, shape, dimensions, and size of the pass-throughregions 40 can be selected to influence kinetics, fluid dynamics, etc.

Embodiments of the cathode 14 can be an electrode configured to generatea reduced chemical product from the input product. For example, thecathode 14 can be configured to reduce the input product by a reductionreaction. The reduced chemical product can be used as output productand/or used to interact with the membrane 22. In some embodiments, thecathode 14 can be configured as a gas-diffusion electrode. This may bedone to facilitate transport of an input product that includes a gas. Insome embodiments, the cathode 14 can include a porous substrate, such ascarbon paper, carbon cloth, electronically conducting metal oxide,polyelectrolyte, ionic liquid, etc. The cathode 14 can have a firstcathode side 14 a. The cathode 14 can have a second cathode side 14 b.Some embodiments can include a cathode catalyst 44 disposed on at leasta portion of any one of the first cathode side 14 a and the secondcathode side 14 b. In some embodiments, the cathode catalyst 44 can be ametal, metal alloy, conductive metal oxide, carbon, or any combinationthereof. Examples of cathode catalysts 44 can include, but are notlimited to gold, silver, copper, indium, bismuth, lead, tin, tellurium,germanium, zinc, or alloys of two or more of these elements, etc. Thecathode catalyst 44 can be configured as a reduction catalyst. Forexample, the cathode catalyst 44 may be configured as a CO₂ reductioncatalyst. A non-limiting example of a CO₂ reduction catalyst can besilver nanoparticles.

In some embodiments, the cathode catalyst 44 can be mixed with a binder,a polymeric electrolyte coating, and/or an ionic liquid. This may bedone in to increase cathode catalyst 44 utilization. For example, thismay provide an ionically conducting pathway to the membrane 22 and/or anelectronically conducting pathway to the cathode 14. For example, duringthe carbon dioxide reduction reaction, protons can be supplied from thecationic side of the membrane 22. When a proton exits the membrane 22,it can be transported to the cathode catalyst 44 through the bindermaterial. The binder material can be a proton conducting material, suchas a sulfonated fluoropolymer, sulfonated polyether, or a polymeric weakacid, for example. With CO₂ being used as input product, the carbondioxide reduction reaction can occur at the interface between thecatalyst surface, the binder surface, and CO₂ gas. This interface may bereferred to as a three-phase boundary. Maximizing the area of thethree-phase boundary can be done to improve the current density of theelectrochemical device 10. In some embodiments, an additive (e.g.,polytetrafluoroethylene) can be added to the cathode catalyst 44 tocontrol wettability of the cathode catalyst layer 44 and/or preventflooding of the cathode 14. The additive can provide hydrophobicity tothe cathode catalyst 44 surface. This may prevent or inhibit H₂O fromcrossing past the cathode catalyst 44. H₂O crossing past the cathodecatalyst 44 may result in water flooding. If water flooding occurs, itcan affect the transport of CO₂ gas to the cathode catalyst 44. This maycause the current density to decrease. Conventional electrochemicaldevices use aqueous catholytes. Under normal operating conditions, thewettability of the binder materials increases with prolonged interactionwith the electrolyte. Thus, after prolonged operation, the cathode canfail due to flooding issues. However, use of a gas phase input product(e.g., using CO₂ gas as the reactant) can reduce flooding concerns. Theflooding concerns can be further reduced by the addition of the additiveto cathode catalyst 44 to control wettability of the cathode catalystlayer 44.

Embodiments of the anode 18 can be an electrode configured to oxidize aninput product. For example, the anode 18 can be configured to oxidizethe input product by an oxidation reaction. In some embodiments, theanode 18 can be configured as a liquid-electrolyte style electrode. Forexample, the electrochemical cell 12 can be configured to operate bytransfer of electrical charge via liquid electrolyte. The electrolytecan be contained within the anode flow medium 20. For example, the anode18, the membrane 22, and the frame 38 can be configured to contain theelectrolyte within the anode flow medium 20. This can include preventingand/or inhibiting the electrolyte from exiting the anode flow medium 20.Embodiments of the electrolyte can include an acidic electrolyte havinga pH less than 7, a basic electrolyte having pH greater than 7, or abuffered electrolyte having a pH at or near 7. Embodiments of the liquidelectrolyte can include an alkali hydroxide solution such as potassiumhydroxide solution (KOH) for example. Other alkali hydroxide solutions(NaOH, RbOH, etc.) can be used. The liquid electrolyte can also includealkali bicarbonate solutions (KHCO3, NaHCO3, etc.)

In some embodiments, the anode 18 can include a porous substrate, suchas carbon paper, carbon cloth, electronically conducting metal oxide,polyelectrolyte, ionic liquid, etc. The anode 18 can have a first anodeside 18 a. The anode 18 can have a second anode side 18 b. Someembodiments can include an anode catalyst 46 disposed on at least aportion of any one of the first anode side 18 a and the second anodeside 18 b. The anode catalyst 46 can be a metal, metal alloy, conductivemetal oxide, carbon, or any combination thereof. Examples of anodecatalysts 46 can include, but are not limited to, iridium oxide,ruthenium alloys or mixed oxides of ruthenium containing iridium and/orplatinum, mixed metal oxides containing cobalt, nickel, iron, manganese,lanthanum, cerium, copper, nickel borate, cobalt phosphate, NiFeOx, etc.The anode catalyst 46 can be configured as an oxidation catalyst. Forexample, the anode catalyst 46 may be configured as a H₂O oxidation orevolution catalyst. A non-limiting example of a H₂O oxidation catalystcan be NiFeOx. Other oxidation catalysts can be used. For exampleoxidation catalysts for any general oxidation reaction, such as oxygenevolution reaction, hydrogen oxidation, chloride oxidation, alcoholoxidation, etc. can be used.

It may be preferred in some embodiments to use a basic electrolyte. Thismay be done so that non-precious metals (e.g., nickel, cobalt, iron,manganese, lanthanum, cerium, copper, etc.) can be used as anodecatalysts 46.

In some embodiments, the anode 18 can be configured as a gas-diffusionelectrode. This may be done to allow the electrochemical device 10 tooperate without a liquid electrolyte. With this embodiment, H₂O in theform of water vapor or steam can be introduced into the anode flowmedium 20 as the input product.

Embodiments of the membrane 22 can include a structure that separatesthe cathode flow medium 16 and/or the cathode 14 from the anode flowmedium 20 and/or the anode 18. The separation can include a physicalseparation, a chemical separation (e.g., chemical isolation), anelectrical separation (e.g., electrical isolation), etc. In someembodiments, the membrane 22 can include a plurality of membranes. Thiscan include forming a bipolar membrane. The bipolar membrane can bestructured as an ion exchange membrane that includes at least one anionexchange layer and at least one cation exchange layer. For example, themembrane 22 can include a cation exchange membrane 22 a and an anionexchange membrane 22 b. The anion exchange membrane 22 b and the cationexchange membrane 22 a may be placed adjacent each other to form aninterface 23, which can also be referred to as a cation-anion exchangejunction or an interface layer. In some embodiments, the anion exchangemembrane 22 b can be adjacent the anode flow medium 20, the anode 18,and/or the anode catalyst 46. In some embodiment, the cation exchangemembrane 22 a can be adjacent the cathode flow medium 16, the cathode14, and/or the cathode catalyst 44. Some embodiments can include aunitary bipolar membrane structure having the anion exchange membrane 22b joined with the cation exchange membrane 22 a. Some embodiments caninclude a separate anion exchange membrane 22 b attached to the cationexchange membrane 22 a. This can include a bipolar membrane 22 having alaminate structure of an anion exchange membrane 22 b and a cationexchange membrane 22 a.

In some embodiments, at least a portion of an interface between theanion exchange membrane 22 b and the cation exchange membrane 22 a caninclude a membrane catalyst. The membrane catalyst can be configured topromote autodissociation of a product. For example, the membranecatalyst can promote autodissociation of H₂O to cause the H₂O todeprotonate into a proton (H⁺) and a hydroxide ion (OH⁻). Examples ofmembrane catalysts can include silicates, amine polymers, graphiteoxides, anolyte solutions (e.g., alkali metal hydroxides or alkali metalcarbonate solutions), etc.

Some embodiments can include providing a cation-exchange polymer film onat least a portion of the anion exchange membrane 22 b. This can includecoating at least a portion of the anion exchange membrane 22 b withNafion (e.g., sulfonated tetrafluoroethylene basedfluoropolymer-copolymer), SPEEK (sulfonated poly(ether ether ketone)),or poly(acrylic acid), for example. This can be done to improve theperformance of the bipolar membrane 22. For example, conventionalbipolar membranes can be thick and resistive. The resistance of themembrane can be an impediment to the performance of the electrolysiscell at high current densities. Thus, the cell performance can beimproved by optimizing the bipolar membrane 22 (e.g. adjusting polymermaterials and fabrication methods) to lower the thickness andresistivity).

In some embodiment, the surface areas at the interface 23 between theanion exchange membrane 22 b and the cation exchange membrane 22 a canbe increased. For example, the surface of any one or both at least aportion of the anion exchange membrane 22 b and at least a portion ofthe cation exchange membrane 22 a at the interface 23 of the two caninclude patterns or other surface features to increase the surface areaof any one or both of them.

Referring to FIG. 5, in at least one embodiment, the electrochemicaldevice 10 can include an electrochemical cell 12. The electrochemicalcell 12 can include a cell first end 30. The electrochemical cell 12 caninclude a cell second end 32. The electrochemical cell 12 may include acathode flow medium 16 at or adjacent the cell first end 30. Theelectrochemical cell 12 may include an anode flow medium 20 at oradjacent the cell second end 32. The electrochemical cell 12 may includea cathode 14 adjacent or within the cathode flow medium 16. The cathode14 can have a first cathode side 14 a and a second cathode side 14 b.The first cathode side 14 a can be adjacent or within the cathode flowmedium 16. The second cathode side 14 b can include a cathode catalyst44. The electrochemical cell 12 may include an anode 18 adjacent orwithin the anode flow medium 20. The anode 18 can have a first anodeside 18 a and a second anode side 18 b. The first anode side 18 a can beadjacent or within the anode flow medium 20. The second anode side 18 bcan include an anode catalyst 46. The electrochemical cell 12 caninclude a membrane 22. The membrane 22 can be positioned between thecathode 14 and the anode 18. The membrane 22 may be configured as abipolar membrane. For example, the membrane can include anion exchangemembrane 22 b and a cation exchange membrane 22 a. The cathode flowmedium 16 can be defined as a volume of space between the cathode 14 andthe cell first end 30. The cathode flow medium 16 can be a carbonmaterial. The cation exchange membrane 22 a can be adjacent the secondcathode side 14 b, the cathode flow medium 16, and/or the cathodecatalyst 44. The anode flow medium 20 can be defined as a volume ofspace between the anode 18 and the cell second end 32. The anode flowmedium 20 can be a carbon material. The anion exchange membrane 22 b canbe adjacent the second anode side 18 b, the anode flow medium 20, and/orthe anode catalyst 46. An interface 23 between the cation exchangemembrane 22 a and the anion exchange membrane 22 b can include amembrane catalyst. In some embodiments, the membrane 22 can beconfigured to separate the cathode 14 and/or cathode flow medium 16 fromthe anode 18 and/or anode flow medium 20.

Embodiments of cathode flow medium 16 can include at least one pathway24. The pathway 24 of the cathode flow medium 16 can be at the interfacebetween the cathode flow medium 16 and the first cathode side 14 a.Embodiments of the anode flow medium can include at least one pathway24. The pathway 24 of the anode flow medium 20 can be at the interfacebetween the anode flow medium 20 and the first anode side 18 a.

Embodiments of the electrochemical cell 12 can include a frame 38. Theframe 38 can be a structure that holds the cathode flow medium 16, thecathode 14, the membrane 22, the anode 18, and the anode flow medium 20of the electrochemical cell 12 together. This can include holding thecathode flow medium 16, the cathode 14, the membrane 22, the anode 18,and the anode flow medium 20 in a serial configuration. The frame 38 canalso be configured to seal at least a portion of the electrochemicalcell 12. The frame 38 can include at least one pass-through region 40.The frame 38 can be configured to seal the electrochemical cell 12except at a pass-through region 40.

The cathode flow medium 16 can include a cell inlet 26 to facilitateintroduction of input product. This can include introduction of inputproduct in a gas phase. The cathode flow medium 16 can include a celloutlet 28 to facilitate removal of output product. The frame 38 caninclude a first pass-through region 40 facilitating introduction ofinput product to a cell inlet 26 of the cathode flow medium 16. Theframe 38 can include a second pass-through region 40 facilitatingremoval of output product from a cell outlet 28 of the cathode flowmedium 16. The anode flow medium 20 can include a cell inlet 26 tofacilitate introduction of electrolyte. The anode flow medium 20 caninclude a cell outlet 28 to facilitate removal of output product. Theframe 38 can include a third pass-through region 40 facilitatingintroduction of electrolyte to a cell inlet 26 of the anode flow medium20. The frame 38 can include a fourth pass-through region 40facilitating removal of output product from a cell outlet 28 of theanode flow medium 20. The anode flow medium 20 can include another cellinlet 26 to facilitate introduction of input product into the anode flowmedium 20. The frame 38 can include a fifth pass-through region 40facilitating introduction of input product to a cell inlet 26 of theanode flow medium 20. The anode flow medium 20 can include another celloutlet 28 to facilitate removal of electrolyte from the anode flowmedium 20. The frame 38 can include a sixth pass-through region 40facilitating removal of electrolyte from a cell outlet 28 of the anodeflow medium 20.

At least one pump can be connected to the pass-through regions 40. Forexample, a first pump can be connected to the first pass-through region40 to facilitate introduction of input product. For example, the firstpump can be configured to introduce CO₂ into the electrochemical device10. A second pump can be connected to the second pass-through region 40to facilitate removal of output product. For example, the second pumpcan be configured to remove CO and/or H₂O from the electrochemicaldevice 10. A third pump can be connected to the third pass-throughregion 40 facilitate introduction of electrolyte. For example, the thirdpump can be configured to introduce electrolyte into the electrochemicaldevice 10. A fourth pump can be connected to the fourth pass-throughregion 40 to facilitate removal of output product. For example, thesecond pump can be configured to O₂ from the electrochemical device 10.A fifth pump can be connected to the fifth pass-through region 40 tofacilitate introduction of input product. For example, the second pumpcan be configured to introduce H₂O into the electrochemical device 10. Asixth pump can be connected to the sixth pass-through region 40 tofacilitate removal of electrolyte. For example, the second pump can beconfigured to remove electrolyte from the electrochemical device 10.Other configurations and number of pumps can be used. For example, someembodiments can use a pump for introduction or removal of multiplefluids, thereby reducing the number of pumps used.

In some embodiments, the input product into the cathode flow medium 16can be CO₂ and/or humidified CO₂. The input product can be transformedinto a reduced chemical product. This can occur within the cathode flowmedium 16. For example, the reaction within the cathode flow medium 16can include a reduction reaction. The reduction reaction can include,for example: CO₂+2H⁺+2e⁻→CO+H₂O. In some embodiments, hydrogen may alsobe generated at the cathode 14. This may be due to competing reductionof protons. CO, H₂O, and/or hydrogen can be caused to exit theelectrochemical cell 12. This can include causing the CO, the H₂O,and/or the hydrogen to exit through a cell outlet 28 and correspondingsecond pass-through region 40 as an output product. The CO, the H₂O,and/or the hydrogen can be captured and/or further processed.

The membrane 22 can supply H⁺ to the cathode flow medium 16 to cause H₂Oto self-ionize via autodissociation to generate hydroxide ions (OH⁻).The membrane 22 can supply the OH⁻ to the anode flow medium 20. The fluxof H⁺ to the cathode flow medium 16 and OH⁻ to the anode flow medium 20can be achieved by generating reverse bias conditions for the bipolarmembrane electrolysis reaction. A constant or stable pH can bemaintained due to the selective transport of H⁻ to the cathode 14 and/orcathode flow medium 16 and OH⁻ to the anode 18 and/or anode flow medium20. A constant or stable pH can be maintained even for extended periodsof time (e.g., 24+ hours). The pH value can be the initial value of theelectrolyte at the beginning of the reaction. The pH level may or maynot be the same for the anode 18 and for the cathode 14. In someembodiments, the pH can be selected by a user depending on the desiredapplication of the electrochemical device 10. Once that value is set,however, the electrochemical device 10 can allow the pH value(s) to beconstant throughout the reaction. For example, the pH for the cathode 14can be set to a first pH level. The pH for the anode 18 can be set to asecond pH level. The first pH level can be the same as or different fromthe second pH level. The electrochemical device 10 can be operated whilemaintaining the first pH level for the cathode 14 and the second pHlevel for the anode 18. Such a configuration can also minimize undesiredcrossover of reduced chemical products from the cathode 14 to the anode18. For example, the bipolar membrane 22 can generate H⁺ and OH⁻ duringthe reaction. The flux of H⁺ and OH⁻, which can be created at theinterface 23 between the anion exchanger and cation exchanger layers 22b, 22 a, is outward towards the electrodes 14, 18. The outward flux ofH⁺ and OH⁻ can prevent ionic and neutral products from crossing overfrom the cathode 14 to the anode 18. In a conventional electrochemicalcell incorporating a monopolar membrane, the flux of ions goes from oneelectrode to the other. This can cause electrodialysis of product anionsand electroosmotic drag of neutral molecules. Electrodialysis of productanions and electroosmotic drag of neutral molecules from the cathode tothe anode can aggravate the crossover problem.

In some embodiments, H₂O can be introduced into the anode flow medium 20as an input product. OH⁻ can also be supplied to the anode flow mediumby the membrane 22. The H₂O and the OH⁻ can be used to generate O₂ as anoutput product. This can occur within the anode flow medium 20 due tointeractions with the electrolyte. For example, the reaction within theanode flow medium 20 can include an oxidation reaction. The oxidationreaction can include, for example: 2OH⁻=1/2O₂+H₂O+2e⁻. The O₂ and/or H₂Ocan be caused to exit the electrochemical cell 12. This can includecausing the O₂ and/or the H₂O to exit through a cell outlet 28 andcorresponding fourth pass-through region 40 as an output product. The O₂and/or the H₂O can be captured and/or further processed.

The e⁻ generated at the anode 18 of the electrochemical cell 12 can bedelivered to the cathode 14 to complete the circuit. (See FIG. 4).

The electrolyte can be introduced into a cell inlet 26 and acorresponding third pass-through region 40. The electrolyte can includean aqueous KOH solution, for example. In some embodiments, theelectrolyte can be removed from the electrochemical cell 12 forprocessing and re-introduction back into the electrochemical cell 12.The electrolyte can be removed through a cell outlet 28 and acorresponding sixth pass-through region 40. The electrolyte can beprocessed to extract O₂ therefrom. The electrolyte can then be directedback into the electrochemical cell 12 via a cell inlet 26 andcorresponding third pass-through region 40. The cycling of electrolytecan occur on a continuous or semi-continuous basis.

Embodiments of the electrochemical device 10 can facilitate use of aninput product in a gas phase. For example, CO₂ can be introduced intothe electrochemical cell 12 as a gas and be further used as a reactantinstead of it being dissolved in the electrolyte, as would be the casewith conventional electrochemical devices. This can reduce or eliminatethe need to provide product separation techniques, as no product isbeing dissolved in an aqueous solution (e.g., the electrolyte). This canfurther minimize or eliminate introduction of contaminants into theelectrolyte. Additionally, because the CO₂ need not be dissolved in theelectrolyte, there is no solubility limitation or slow mass transportissues associated with dissolved CO₂ in liquid electrolyte to act as anoperational constraint, as would be the case with conventionalelectrochemical devices. For example, CO₂ has low solubility in liquidelectrolytes suitable for CO₂ electrolyzer devices, which may lead toreduced mass transport of CO₂ molecules in the liquid electrolyte.

Embodiments of the electrochemical device 10 can facilitate stableoperation of the electrochemical cell 12 at high current density andhigh faradaic efficiency. For example, embodiments of theelectrochemical device 10 can operate at current densities within arange from 100 mili-Ampere per square centimeter (mA/cm²) to 1 A/cm² andwith at least 80% faradaic efficiency. Such current densities andfaradaic efficiencies can be sustained with electrolyte selectivity(e.g., passage of ions) of at least 40%. Embodiments of theelectrochemical device 10 can operate in a stable manner for over 24hours (e.g., no significant decay in current density and faradaicefficiency).

In some embodiments, the current densities can be improved by optimizingthe thickness and composition of the bipolar membrane 22 and thecomposition and dispersion of the catalysts 44, 46. FIG. 11 shows theperformance improvement (e.g., current density) of an embodiment of thebipolar membrane 22 via optimization methods, as compared to aconventional bipolar membrane.

In some embodiments, product selectivity can be tuned by selectingdifferent cathode catalysts 44. This can be done to generate high valuechemicals (e.g., methanol, ethylene, DME, formate, methane, methanol,ethylene glycol, butanol, etc.) in addition to or in the alternative togenerating carbon monoxide. For example, gold and/or silver can be usedas cathode catalysts 44 to generate carbon monoxide. Lead, bismuth,and/or tin can be used as cathode catalysts 44 to generate formate.Copper-based cathode catalysts 44 can be used to generate methanol,methane, ethylene, ethylene glycol, butanol, etc. Generally, acopper-based cathode catalyst 44 can be in a specific nanostructure ordisplay certain crystal facets to facilitate tailoring it towards aparticular product.

Embodiments of the electrochemical device 10 can be used as part of anair purification unit. For example, the air purification unit caninclude at least one electrochemical device 10 configured as a CO₂electrolyzer. The air purification unit can be configured to consume CO₂at the cathode 14 and generate O₂ at the anode 18. When used in aconfined space (e.g., submarine, space vehicles, energy-efficient officebuildings, etc.), the air purification unit can be used to replace CO₂with O₂, thereby purifying air.

It is contemplated for the operating temperate of an embodiment of theelectrochemical device 10 to range from 20 degrees Celsius to 130degrees Celsius. In at least one embodiment, the electrochemical device10 can operate under ambient conditions (25° C. and 1 atmosphericpressure). In some embodiments, the operating temperature can vary fromroom temperature (approximately 25 degrees Celsius) to up to 80 degreesCelsius. In some embodiments, the operating temperature can be higherthan 80 degrees Celsius, and even up to 130 degrees Celsius. Forexample, inorganic additives may be used in polymer membranes of thebipolar membrane 22, which can extend the useful range up to 130Celsius.

It is contemplated for the electrochemical device 10 to have betterperformance as the operating temperature increases. For example, higheroperating temperatures can improve the kinetics of ion transport. Higheroperating temperatures can improve catalytic activity of the anodecatalyst 46 and/or the cathode catalyst 44. Operating temperatures atwhich the membrane 22 begins to dehydrate, however, may degradeperformance.

In addition, the pressure of the CO₂ gas may be increased to increasethe current density and selectivity for CO₂ reduction. For example, theoperating pressure can range from 1 atmosphere pressure to 100atmospheres pressure. Some embodiments can use an operating pressuregreater than 100 atmospheres (e.g., as high as the mechanical structureof the electrochemical device 10 will hold). Generally, the higher thepressure, the better is the performance.

Other operating parameters can include flow rate. For example, the flowrate of the input product can be determined through the current densityrequired for the specific application. For CO₂ gas, for example, theflow rate can be estimated to be within a range from 0 to 100liters/minute.

In some embodiments, the anode catalyst 46 and/or the cathode catalyst44 may be hot-pressed together with the bipolar membrane 22 to create aunitary membrane-electrode assembly. Hot-pressing the cathode catalyst44 and/or the anode catalyst 46 with the bipolar membrane 22 can createa more intimate contact between the surfaces. This may facilitate thetransport of protons. It may also be more beneficial for processesinvolved with fabricating membrane-electrode assemblies.

EXAMPLES

In a non-limiting example, an embodiment of the electrochemical device10 was created using a gas diffusion cathode 14. The cathode 14 includeda piece of Toray carbon paper (Toray® TGP-H-120). Silver nanoparticles(100 nm diameter, Sigma Aldrich®) were use as the CO₂ reduction catalyst44. Cathode catalyst ink 44 was made by mixing 8 miligrams of silvernanoparticles with 200 microliters of isopropyl alcohol, 200 microlitersof deionized water (18.2 Me), and 15 microliters of 5% Nafion(sulfonated tetrafluoroethylene based fluoropolymer-copolymer solution),which was sonicated for 10 minutes. The cathode catalyst ink 44 was thenpainted onto the carbon paper at a typical loading of about 5 miligramsper square centimeter (mg/cm²). The anode 18 included a piece of Toraycarbon paper (Toray® TGP-H-120). NiFeOx was used as the anode catalyst46. NiFeOx was electrodeposited onto the carbon paper.

The anode 18 and cathode 14 were assembled together with a bipolarmembrane 22. KOH solution (0.1 M) was delivered to the anode flow medium20 via a peristaltic pump as the anolyte. Gaseous CO₂ was humidifiedthrough a water bubbler and then flowed into the cathode flow medium 16at 20 standard cubic centimeters per minute (sccm). FIG. 6 showsstability data for an embodiment of the electrochemical device operatingunder constant 2.8 Volts for 24+ hours. FIG. 6 indicates that both thecurrent and the faradaic efficiency of CO production were stable for 24+hours. These data demonstrate the increased stability of CO₂electrolysis in an embodiment of the electrochemical device 10. Bothcurrent density and faradaic efficiency may be improved through furtherengineering optimization.

FIG. 7 compares cell potential over time for a conventionalelectrochemical device using a nafion cation exchange membrane and anembodiment of the electrochemical device 10. Both devices were equippedwith the same cathode and anode catalysts. Both were operated a constantcurrent of 50 mA/cm². Both were supplied with humidified CO₂. Theconventional electrochemical device failed after 8 hours, whereas anembodiment of inventive electrochemical device 10 showed stable currentdensity for the entire duration of the test. Performance degradation forthe conventional electrochemical device may be due to the changes in pHat the anode and cathode. It is contemplated that an electrochemicaldevice 10 using a bipolar membrane 22 should operate indefinitely aslong as the membrane 22 does not degrade.

As noted herein, use of gaseous CO₂ as the reactant instead of CO₂dissolved in liquid electrolyte can provide an advantage. For example,the solubility of CO₂ is about 34 miliMolar in water at 1 atmosphere.Such solutions may support a maximum current density of about 20 mA/cm²for conventional devices. The use of gaseous CO₂, as with an embodimentof the inventive electrochemical device 10 however, can obviate thesolubility limit of CO₂ in water. FIG. 8 shows a current-voltage curvefor an embodiment of the electrochemical device 10 operating at highcurrent density and at 60 degrees Celsius with the introduction ofhumidified gas-phase CO₂ at the cathode 14. NiFeOx was used as the anodecatalyst 46. A 0.1 M aqueous KOH solution was used as the liquidelectrolyte. A bismuth/1-Butyl-3-methylimidazoliumtrifluoromethanesulfonate (BMIM+OTf−) was used as the cathode catalyst44. The cathode 14 was gas-fed with humidified CO₂. The gradual levelingoff of the current at high cell potential suggests that the masstransport limit with the bipolar membrane 22 is in excess of 200 mA/cm².As noted herein, other bipolar membrane designs can support currentdensities as high as 1 A/cm².

Additionally, trace amounts of impurities in liquid electrolytes cancause degradation of electrode selectivity. This can be due to catalyticmetals present in the catholyte being deposited on the cathode 14, whichmay promote undesired hydrogen evolution. FIGS. 9 and 10 show faradaicefficiency plots for a conventional device using a silver catalyst in0.5 M of KHCO₃ for the cathode electrolyte and 0.1M KOH for the anodeelectrolyte. The device was configured as a bipolar membraneelectrolyzer with an aqueous bicarbonate catholyte. These datademonstrate the degradation of electrode selectivity. For example, thegraphs show a decline in faradaic efficiency for CO production with anincrease for hydrogen production. The most likely cause for this is thedeposition of impurity ions from the electrolyte onto the catalystsurface and their promotion of the competing hydrogen evolutionreaction. However, as noted herein, use of embodiments ofelectrochemical device 10 can reduce or eliminate introduction ofimpurities in the liquid electrolyte. For example, use of CO₂ as the gasreactant can allow for the elimination of dissolution of CO₂ in theelectrolyte.

As noted herein embodiments of the device 10 can be configured toreduce, inhibit, and/or eliminate product crossover. For example,embodiments of the bipolar membrane 22 can configured to provide a fluxof H⁺ to the cathode 14 and a flux of OH⁻ to the anode 18. Withconventional designs, chemicals can crossover from one electrode toanother even though they are separated by an ion exchange membrane. Thecrossover can be driven by electrokinetic effects (e.g., when ions passthrough an ion exchange membrane under applied current, they drag alongother molecules). An example of this is illustrated in FIG. 12. Incontrast, conventional designs result in desirable chemical productsthat are generated from CO₂ reduction at the cathode being dragged tothe anode and oxidized, which lowers the overall energy efficiency ofthe device. Embodiments of the device 10 having an embodiment of thebipolar membrane 22 can be configured to generate ionic movements thatare different from those of conventional devices. With an embodiment ofthe device 10 under applied current conditions, ions move outward towardthe electrodes 14, 18. An example of this is illustrated in FIG. 13.

Due to the bipolar membrane 22 design, the electrokinetic effects can beused to push product chemicals outward (e.g. away from the membrane 22and interface 23), and thus reduce, inhibit, and/or prevent theseproducts from crossing over the membrane 22. This can not only preventproducts crossover, but also improve the stability of a device 10configured as an electrochemical cell 12. This can be a significantdifference as compared to conventional designs, that result in theelectrokinetic effects of the design causing the product chemicals tomove from one electrode 14, 18 to the other,

Embodiments of the device 10 can include a bipolar membrane 22configured to generate ionic movements that are different from those ofconventional devices. For example, an embodiment of the device 10 can beconfigured so that, under applied current conditions, ions move outwardtoward the electrodes 14, 18. As noted herein, embodiments of thebipolar membrane 22 can be configured to supply H+ to the cathode flowmedium 16 to cause H₂O to self-ionize via autodissociation to generatehydroxide ions (OH⁻). The bipolar membrane 22 can supply the OH⁻ to theanode flow medium 20. The flux of H⁺ to the cathode flow medium 16 andOH⁻ to the anode flow medium 20 can be achieved by generating reversebias conditions for the bipolar membrane 22 electrolysis reaction. Dueto the bipolar membrane 22 design, the electrokinetic effects, insteadof carrying product chemicals from one electrode 14, 18 to the other,push product chemicals outward, and thus reduce, inhibit, and/or preventproducts from crossing over the membrane 22.

In at least one embodiment, the bipolar membrane 22 can include at leastone anion exchange layer 22 b and at least one cation exchange layer 22a joined together at the interface, which may also be considered aninterfacial layer. The interface 23 can be configured to catalyze theautodissociation of H₂O. Under reverse bias conditions, H⁺ and OH⁻ ionscan be generated in the catalytic layers 22 a, 22 b and be drivenoutward. The flux of H⁺ in the bipolar membrane 22 can oppose thedirection of product crossover from the cathode 14 to the anode 18 of anelectrolytic cell 12. The outward fluxes of H⁺ and OH⁻ generated inembodiments of the bipolar membrane 22 can inhibit the crossover of bothanionic and neutral products, even with membranes 22 that contain highsurface area junctions. In some embodiments, devices 10 configured as anelectrochemical cell 12 having an embodiment of the bipolar membrane 22can operate continuously with high faradaic and energy efficiency, andwith current densities in the 1-2 A cm⁻² range.

A comparison of the chemical crossover between a conventional anionexchange membrane (AEM) and an embodiment of the bipolar membrane 22 fordifferent output products (e.g., formate methanol, and ethanol) can beseen in FIG. 14. FIG. 14 shows crossover of formate, methanol, andethanol versus time in electrochemical cells having an AEM membrane andan embodiment of the bipolar membrane 22. 0.5 M KHCO3 was used as theelectrolyte on both the cathode and anode sides of the electrochemicalcell 12. 0.15 M formate, methanol, or ethanol was added to thecatholyte, and 50 mA constant current was applied. Concentrations on thecathode and anode sides are plotted as percentages, normalized to theinitial concentration.

It can be seen that an approximate 15% decrease of concentration offormate in the cathode occurred with the AEM device. In addition, theAEM device utilized a thicker AEM. Embodiments of the device 10 having abipolar membrane 22, however, experienced almost no change inconcentration. Similar behaviors were observed with methanol and ethanolforms of electrolyte. Details of the crossover experiments are providedbelow.

The crossover rates of formate, methanol, and ethanol, which can bedesirable CO₂ reduction products, were compared in devices containingAEMs and bipolar membranes 22. The crossover of formate, an anionic CO₂reduction product, occurs by electromigration through AEMs, and its rateincreases linearly with current density. Crossover of electroneutralmethanol or ethanol through AEMs occurs to a lesser extent through bothdiffusion and electroosmotic drag, the latter increasing with currentdensity in AEMs. In contrast, the outward fluxes of H⁺ and OH⁻ generatedin embodiments of the bipolar membranes 22 can inhibit the crossover ofboth anionic and neutral products, even with membranes 22 that containhigh surface area junctions. Calculated electroosmotic drag coefficientsfor each of the neutral products confirm the better performance ofbipolar membranes 22 in terms of product losses.

Embodiments of the device 10 can be configured to achieve operatingparameters that are conducive to provide effective electrolyzer units(e.g., operate continuously with high faradaic and energy efficiency,and with current densities in the 1-2 A cm⁻² range). At such highcurrent densities, product crossover should be considered as a lossmechanism. Because crossover is primarily driven by electrokineticeffects, its rate increases with increasing current density. Thus, it iscontemplated for embodiments of the bipolar membrane 22 to be used togenerate devices 10 that can meet desired operating requirements of CO₂electrolysis cells while minimizing product crossover.

The bipolar membrane can also be referred to herein as “BPM”.Embodiments of the bipolar membrane 22 can include anion exchange layers22 b and cation exchange layers 22 a joined together at an interface 23.The interface 23 can be an interfacial layer that is configured tocatalyze the autodissociation of H₂O. Under reverse bias conditions, H+and OH− ions can be generated in the catalytic layers 22 a, 22 b and bedriven outward. The flux of H⁺ in the BPM 22 opposes the direction ofproduct crossover from the cathode 14 to the anode 18 of an electrolyticcell 12. This electromigration of anionic products, as well as transportof neutral molecules by electroosmotic drag, can be minimized in anelectrolytic cell 12 having an embodiment of the BPM 22.

Experiments were conducted to compare crossover through conventionalelectrochemical cells having AEMs and BPMs 22. Cation-exchange membranessuch as Nafion were eliminated from this study because earlierexperiments have shown that they are a poor choice for maintaining pHbalance and minimizing crossover in CO₂ electrolysis. AEMs are moretypically the monopolar membrane of choice in CO₂ reuction reaction (RR)studies because of the high solubility of CO₂ (as bicarbonate, HCO₃ ⁻)under neutral and mildly basic conditions. In addition to electroneutralproducts, formate, acetate, and oxalate are anionic products that can begenerated by the electrolysis of HCO₃ ⁻. Experimental results show thatthe crossover of formate is significant when AEMs are used, especiallyat high current densities, and that neutral molecules such as methanoland ethanol exhibit crossover to a lesser extent. In contrast, withembodiments of the BPM 22, substantially less crossover of both anionicand neutral molecules were observed. (See FIG. 14).

The rates of anionic and neutral molecule crossover were measured in ahydrogen cell with 80 mL of 0.5 m KHCO₃ as the electrolyte in both thecathode 14 and anode 18 compartments. In addition, crossover rates weremeasured with a BPM 22 containing a two-dimensional junction structure(can be referred to as a 2D BPM 22) and a BPM 22 containing athree-dimensional junction structure (can be referred to as a 3D BPM22). Embodiments of the 2D BPM 22 can have a 2D planar junctioninterface 23 between the anion exchange layer 22 b and the cationexchange layer 22 a. Embodiments of the 3D BPM 22 can contain a networkof interpenetrating anion- and cation-exchange layers 22 b, 22 a.Embodiments of the 3D BPM 22 can be configured as an extended waterdissociation junction so as to sustain water electrolysis at currentdensities up to 1 A cm⁻². Formate, methanol, or ethanol was added at aninitial concentration of 0.15 m to the catholyte of the cell in order tosimulate conversion of ≈¼ of the HCO₃— reactant to products, and theconcentration of these molecules was monitored periodically by 1H NMR onboth the cathode 14 and anode 18 sides of the cell 12 duringelectrolysis at constant current.

FIG. 14 shows the change in concentration versus time, normalized to theinitial concentration of 0.15 m, as a 50 mA constant current was appliedto the electrochemical cell 12. The exposed area of the membrane wasabout 1.1 cm² in all experiments. For formate, a linear decrease inconcentration with time was observed at the cathode, up to ≈15% in 4hours of electrolysis with the AEM. A corresponding increase inconcentration was found at the anode, indicating that formate passedthrough the membrane to the anode. This is expected since formate issimilar in size to bicarbonate and can pass through the AEM byelectromigration. The concentration of formate in the anolyte leveledout at ≈0.015 m (10% of the initial catholyte concentration), presumablybecause it was oxidized back to bicarbonate as it reached the anode.Under these conditions, the transference number for formate in the AEMis about 0.24 and the formate to total anion ratio is 0.23, meaning thatthe AEM is unselective for formate versus HCO₃ ⁻ ions. In contrast, thecrossover rate of formate was about 17 times lower when BPM 22 or 3D BPM22 was used under the same conditions.

Methanol and ethanol are neutral molecules that do not electromigrate,but are susceptible to transport across membranes by both simplediffusion and electroosmotic drag (see FIG. 12). A loss of about 1% ofmethanol and 0.8% of ethanol were observed at the cathode, andcorresponding increases in their concentrations were found at the anodeafter 4 hours of electrolysis in the AEM cell. Under these conditions,the permeation rates for methanol and ethanol were 0.025 and 0.017 mmolh⁻¹, respectively, at an applied current of 50 mA. The flux ratio (χ) ofneutral molecules (methanol and ethanol) relative to ions can becalculated according to:

$x = \frac{n_{chem}}{n_{ion}}$

where n_(chem) and n_(ion) are the number of moles of the neutralmolecule and the number of moles of ions that pass through the membrane,respectively. Table 1 shows these flux ratios for methanol and ethanolin AEM- and BPM-based cells.

TABLE 1 Flux ratios for methanol and ethanol in AEM-and BPM-based cellsAEM-based cell BPM-based cell 12 Methanol 0.014 0.007 Ethanol 0.0090.006

As noted above, x can contain contributions from both diffusion andelectroosmotic drag. Typical χ values for methanol in Nafion-baseddirect methanol fuel cells range from about 1 to 10. The much lowervalues observed here are likely due to the fact that the ion-moleculeinteraction of methanol or ethanol with protons in Nafion is muchstronger than it is with bicarbonate in the AEM or BPM 22. Theelectroosmotic drag coefficient as well as the diffusion constant ofmethanol is known to be much smaller in AEMs than in Nafion membranes.For both methanol and ethanol cases, the flux ratios are significantlylower in the BPM 22 than in the AEM. The difference appears to belargely a consequence of electroosmosis in the case of the AEM, becausein that case the crossover rate increases linearly with current density(see FIG. 15).

While only about 1.5% of the methanol crosses over in 2 hours at acurrent of 200 mA (≈180 mA cm⁻²), with AEM-based electrolyzers thatoperate at current densities of 1-2 A cm⁻², electroosmosis would resultin significant crossover losses. In contrast, the methanol crossoverrate increases only slightly with increasing current density in theBPM-based cell 12, where the ion flux is directed outward rather thanacross the membrane.

FIG. 15 shows that the crossover rate of formate increases linearly inthe AEM cell, as expected for a transport process that is dominated byanion electromigration. In contrast, the crossover flux of formateremains low in the BPM-based cell 12, even at high current density. Thethicknesses of the AEM and BPM 22 used in this study were 170 and 140μm, respectively. Given that the thicknesses are similar, and thecrossover flux is much higher under higher applied current conditions,it can be concluded that electrokinetic transport mechanisms are muchmore important than permeation.

Embodiments of the 2D BPM 22 have a 2D planar junction interface 23between the anion exchange layer 22 b and the cation exchange layer 22a. In contrast, the 3D BPM 22, prepared by electrospinning, contains anetwork of interpenetrating anion- and cation-exchange polymer layers 22b, 22 a at the catalytic junction. Because the favorable I-Vcharacteristics of the 3D BPM 22 depend on the high surface area of thejunction, it may be beneficial to know whether crossover rates arehigher in the 3D BPM 22 than in 2D BPMs 22. Despite having lower overallthickness and larger interfacial layer, or interface 23, FIG. 14 showsthat the 3D BPM 22 has very similar rates of formate and methanolcrossover as the 2D BPM 22. The crossover rates, which are mostaccurately measured by the appearance of products on the anode 18 sideof the cell 12, are the same within one standard deviation for the twotypes of membranes (the 2D and the 3D BPMs 22). This indicates that thecrossover rate does not depend on the interfacial area of any embodimentof the BPM 22. Instead, the rate is limited by the permeation ofmolecules through the relatively thick anion- and cation exchange layers22 b, 22 a in both kinds of BPM membranes 22.

Referring to FIG. 16, crossover rates of formate and methanol at zerocurrent density were also measured in order to eliminate electrokineticeffects and quantify the flux of molecules that can permeate the AEM orBPM 22 by diffusion. FIG. 16 shows that there is a very low rate ofmethanol crossover through either the AEM or BPM 22. Formate does showmeasurable crossover rate through the AEM, as indicated by aconcentration that increases linearly with time on the anode 18 side ofthe cell 12. At the pH of the catholyte (pH=7.3) and anolyte solutions(pH=8.2), formate (pKa=3.75) exists almost entirely as the formateanion, and neutral formic acid should not contribute significantly tothe flux of formate across the membrane. At the solution concentrationsused (0.15 m formate and 0.5 m HCO₃ ⁻), formate will occupy asignificant fraction of the anion-exchange sites in the AEM, and itsdiffusion across the membrane (as a neutral ion pair) will be limited bythe concentration and diffusion coefficient of the K⁺ co-ion in the AEM.A similar diffusion mechanism can occur in the anion-exchange layer 22 bof the BPM 22, which will contain both formate and K+ cations under zerocurrent conditions. However, the formate ion concentration should be lowin the cation-exchange layer 22 a of the BPM 22, and the concentrationsof both formate and K⁺ should be low in the electroneutral interfaciallayer. The result is that diffusion of formate as a neutral ion pairwith K⁺ is significantly slower across the BPM 22 than it is across theAEM. This observation however is not terribly relevant to the BPM 22under electrolytic conditions, where the cation- and anion-exchangepolymer layers 22 b, 22 a are charge-compensated predominantly by H⁺ andOH⁻ ions.

Earlier studies have shown that the use of either AEMs and Nafionmembranes can be problematic with gas diffusion cathodes that are fed bygaseous CO₂, because the pH of the cathode and anode shift undercontinuous operation. In CO₂ electrolyzers that employ an aqueouscatholyte, bicarbonate salts are typically the electrolyte of choicebecause of the high solubility of CO₂, and AEMs are used because the pHcan be balanced in continuous operation by recycling CO₂ (liberated byoxidation of HCO3⁻) from the anode to the cathode. However, theexperiments demonstrated show that in such AEM-based electrolyzers thecrossover of anionic products such as formate occurs even at relativelylow current density, and the crossover of neutral products such asmethanol can become problematic at high current densities currentlyemployed in water electrolyzers. These experiments further demonstratethat embodiments of the BPM 22 can sustain high current densities andcan inhibit crossover of both anionic and neutral products of CO₂electrolysis.

In some embodiments, the water dissociation reaction at the interface 23of an embodiment of the BPM 22 can be tuned. This can be done by addingat least one catalyst layer 25 to an embodiment of the bipolar membrane22. For example, an embodiment of the bipolar member 22 can include acation exchange layer 22 a and an anion exchange layer 22 b. In someembodiments, the cation exchange layer 22 a can be adjacent the anionexchange layer 22 b to form a cation-anion exchange junction region atthe interface 23. Within the cation-anion exchange junction region, acatalyst 25 can be deposited on at least a portion of the anion exchangelayer 22 b to form an interfacial catalyst layer 25. Some embodimentscan have a plurality of interfacial catalyst layers 25. Embodiments ofthe catalyst 25 can be graphite oxide (GO), polymeric amines, clayplatelets, transition metal phophate particles, or transition metaloxide particles, for example. Some embodiments can involve depositingthe catalyst 25 via a layer-by-layer assembly technique (e.g., vialamination). A layer-by-layer assembly technique can involve serialexposure of the membrane to solutions of polycations and polyanions,which can facilitate precise control of layer thicknesses. Otherassembly techniques can be spin-coating and dip coating. These may beadvantageous because they can require fewer processing steps. The cationexchange layer 22 a can be deposited on the catalyst 25. Adding acatalyst layer 25 can be done to balance the effects of an appliedelectric field and the interfacial catalysis 25. For example,embodiments of the bipolar membrane 22 having at least one interfacialcatalyst layers 25 can decrease the electric field intensity across theinterface 23. Damping of the electric field in can be the result of ahigher water dissociation product (H⁺/OH⁻) flux, which can neutralizesthe net charge density of the cation exchange layer 22 a and anionexchange layer 22 b. Thus, the amount and type of catalyst 25 added orthe number of catalyst layers 25 in the cation-anion exchange junctionat the interface 23 can be optimized to tune the performance of anembodiments of the bipolar membrane 22.

In some embodiments, the water dissociation reaction at the interface 23of an embodiment of the BPM 22 can be tuned. This can be done byincorporating different layers of graphene oxide (GO) catalyst 25 tobalance the role of electric field and the interfacial catalysis. Forexample, an embodiment of the BPM 22 can be formed by a lamination of acation exchange layer 22 a and an anion exchange layer 22 b. Uponapplication of a reverse bias, the ordinarily slow water dissociationreaction at the cation-anion exchange junction at the interface 23 ofthe BPM 22 can be dramatically accelerated by the large electric fieldat the interface 23 and by the presence of catalyst.

Experiments using electrochemical impedance spectroscopy (EIS) haveconfirmed that a counterbalanced role of the electric field and thejunction catalyst in accelerating water dissociation in an embodiment ofthe BPM 22 can be achieved. Experimental embodiment of BPMs 22 wereprepared from a crosslinked anion exchange layer 22 b and a Nafioncation exchange layer 22 a, with a graphite oxide (GO) catalyst 25deposited at the cation-anion exchange junction at interface 23 usinglayer-by-layer (LBL) assembly techniques. BPMs 22 with an interfacialcatalyst layer 25 were found to have smaller electric fields at theinterface compared to samples with no added catalyst 25. A comprehensivenumerical simulation model showed that the damping of the electric fieldin BPMs 22 with a catalyst layer 25 is a result of a higher waterdissociation product (H⁺/OH⁻) flux, which neutralizes the net chargedensity of the cation exchange layer 22 a and anion exchange layer 22 b.This conclusion is further substantiated by EIS studies of ahigh-performance 3D BPM 22 that shows a low electric field due to thefacile catalytic generation and transport of H⁺ and OH⁻. Numericalmodeling of these effects in the BPM 22 provides a prescription fordesigning membranes that function at lower overpotential.

It is contemplated that the rate of water dissociation at thecation-anion exchange junction at interface 23 can limit the energyefficiency of BPM-based electrolysis devices 10. This rate isdramatically increased by the high electric field and the presence ofcatalysts 25 in the cation-anion exchange junction region at interface23. The combined electrochemical impedance and simulation study revealsthat the electric field across the cation-anion exchange junction isweakened by the H⁺/OH⁻ flux from catalyzed water dissociation, whichpartially neutralizes the unbalanced fixed charges on the anion exchangelayer 22 b and the cation exchange layer 22 a. The amount of catalyst 25in the cation-anion exchange junction at the interface 23 can beoptimized to tune the performance of embodiments of the BPM 22.

It is contemplated for proton transport to be a vital process inembodiments of the electrochemical cell 12 because cathodic electrontransfer is accompanied by the consumption of protons. Membraneseparators are typically incorporated into the electrolysis system toallow for selective passage of electrolyte ions and the separation ofthe cathodic and anodic products. Mass transfer in membrane separatorscan induce additional resistance and can result in a transmembrane pHgradient, compromising the energy efficiency of the system. Althoughconventional electrolyzers normally operate under strongly acidic orbasic conditions to minimize series resistance, pH neutral electrolytesare advantageous for some oxygen evolution reaction (OER) catalysts thatcontain only earth-abundant elements. Previous studies of electrolyticcells with buffer-based electrolytes and conventional anion- and cationexchange membranes (A/CEM) have suggested that a 4300 mV pH gradientdevelops across conventional A/CEM separators under DC polarization,which is only partially mitigated by back diffusion if electroneutralbuffers are used.

Embodiment of the BPM 22 having oppositely charged anion exchange andcation exchange layers 22 b, 22 a can allow for the separation of acidicand basic solutions in the cathode 14 and anode 18 compartments,respectively, thus providing optimal pH conditions catalysts. Inaddition, under reverse bias, i.e., with the cation exchange layer 22 afacing the cathode 14, the water dissociation reaction that occurs inthe membrane 22 replenishes the cathode 14 and anode 18 with H⁺ and OH⁻,respectively, minimizing electrolyte adjustments. Moreover, the pHgradient at the BPM/electrolyte interface is mitigated due to thepredominance of H⁺/OH⁻ species inside the anion exchange and cationexchange layers 22 b, 22 a, which match the principal charge carriers inthe electrolyte. As a result, most of the cross-membrane potential dropoccurs at the cation-anion exchange junction positioned at the interface23. Thus, it can be beneficial to tailor the structure of the BPM 22 atthe cation-anion exchange junction.

Referring to FIG. 17, the large electric field created under reversebias and the catalyst 25 in the cation-anion exchange junction region ofthe interface 23 of the BPM 22 can dramatically enhance the rate ofwater dissociation at the cation-anion exchange junction of theinterface 23. Experiments were conducted to explore the correlationbetween the electric field and the junction catalyst 25 in promoting thewater dissociation reaction in an embodiment of the BPM 22. Tosystematically and controllably adjust the structure at the cation-anionexchange junction of the interface 23, a BPM 22 with a lightlycrosslinked anion exchange layer 22 b with a flat surface was generated.A catalyst 25 was then deposited on at least a portion of the anionexchange layer 22 b with a flat surface. The catalyst 25 was graphiteoxide (GO). The deposition techniques involved layer-by-layer (LBL)assembly methods. A thin film of Nafion from a solution indimethylformamide (DMF) was deposited on the catalyst 25 as the cationexchange layer 22 a.

Exemplary BPMs 22 with one layer of GO and four layers of GO as thejunction catalyst 25 were tested by using electrochemical impedancespectroscopy (EIS) and compared against a BPM 22 without a GO catalyst25. Results demonstrate that incorporating the catalyst 25 decreases theelectric field intensity across the BPM cation-anion exchange junctionof the interface 23. A numerical simulation model taking into accountthe ionic transport, electrostatics, and electric field-dependentdissociation reaction confirmed the experimental findings. Furthermore,EIS measurements on a BPM 22 with a 3D junction substantiated theconclusions from the numerical model. The BPM 22 with a GO catalyst 25showed a significantly lower cross-membrane potential drop than the BPMwithout a GO catalyst 25 at 4100 mA cm² current density and hadcomparable stability over a 10 hour test.

In making the samples, Nafion dissolved in DMF was deposited at 120° C.The relatively high processing temperature and the DMF solventalleviated the rod-like aggregation that can occur in lower temperaturealcohol/water Nafion dispersions. GO layers 25 were deposited as thejunction catalyst with poly-dialkyldimethylammonium (PDDA) as thepolycation using layer-by-layer assembly techniques to allow for precisecontrol over the interfacial structure of the interface 23. From thecross-sectional scanning electron microscope (SEM) images of the BPM 22,an anion exchange layer 22 b of about 100 mm and a cation exchange layer22 a of about 40 mm thickness can be clearly distinguished, whereas theGO interfacial layer 25 was too thin to be imaged by this technique.

EIS measurements were performed while systematically varying the reversebias on the 1GO layer 25 samples (samples having only one catalyst layer25) and the 4GO layer 25 samples (samples having four catalyst layers25) and compared with results from a BPM 22 fabricated without acatalyst 25 0GO layer (samples having no catalyst layer 25) as acontrol. EIS measurements were carried out in a four-electrode cell inwhich current was applied through outer working and counter electrodesand the potential was measured between Ag/AgCl (3 M NaCl) referenceelectrodes (RE) positioned close to the faces of the BPM 22 viaHaber-Luggin capillaries. This arrangement minimized the effects ofsolution resistance and eliminated the overpotentials for the HER andOER, as well as the electrode double-layer capacitance at the working(WE) and counter electrodes (CE) in the EIS measurements and the J-Ecurves. An AEM was placed between the CE and the other REs for the samereason. A DC current was initially applied via the working and counterelectrodes to reach steady-state conditions, and EIS data were thenacquired by applying a small amplitude AC signal.

An equivalent circuit developed from the neutral layer model was used tofit all the experimental spectra as it was contemplated for (1) theincorporation of the GO catalyst layer 25 into the BPM 22 to have beenbetter described by a model that treated the BPM interfacial layer 25explicitly and (2) an abrupt junction was unlikely to exist in BPMs 22that were prepared. The overall impedance was then modeled by the seriesconnection of a Gerischer element, an Ohmic resistor representing themembrane and bulk electrolyte, and a block consisting of a resistor anda capacitor. The quality of the EIS fitting to the equivalent circuitwas confirmed by noting the parameter w2, showing a value of B0.01 andB0.003 for the 4GO and 0GO BPM, respectively.

FIG. 18 compares J-E curves of the BPMs 22 under reverse bias rangingfrom 0.6 V to 1.5 V. All BPMs 22 had similar co-ion leakage currentdensity, which is below 0.5 mA cm², as indicated by the flat portion ofthe J-E curve between 0.6 and 0.7 V. Above 0.75 V, the current increasessignificantly as the dominant current-carrying ions in the CEL and AELbecome H⁺ and OH⁻, respectively. The dissociation of water can bedescribed by the following reaction.

H₂O(l)=H⁺(aq)+OH⁻(aq)

The dissociation of water has a formal potential (at unit activity of H⁺and OH⁻) of 0.83 V at room temperature, close to observed onset bias.Beyond the onset potential, the 4GO BPM 22 has the lowest potential at agiven current density, followed by the 1GO BPM, which has much highercurrent density than the 0GO BPM within the studied reverse bias range.The water dissociation rate constants, kd, of all BPMs 22 increase withincreasing bias (See FIG. 19). This suggests that water dissociation isenhanced by the electric field, irrespective of the presence of acatalyst 25. To be specific, the 0GO and 1GO BPMs 22 show appreciableincreases in kd at voltages between 0.8 to 0.9 V (see FIG. 19). Incontrast, kd for the 4GO BPM 22 is relatively large at low reverse biasand increases with increasing voltage.

FIG. 20 shows the reaction resistance, Rw, as extracted from theelectric double layer (EDL) in the EIS equivalent circuit. The BPMs 22fabricated from the anion exchange layer 22 b plus Nafion share the sametrend in Rw, i.e., that it decreases as voltage increases and graduallyconverges to a plateau. The flat portion of the curve corresponds to aquasi-equilibrium region for the water dissociation reaction, where theforward dissociation and backward neutralization reaction rates canceleach other and are equal to the exchange current. Before reaching thequasi-equilibrium region, the dissociation reaction is largelysuppressed because of the fast backward acid—base neutralizationreaction. An increase in reverse bias helps promote the dissociationreaction, thus decreasing Rw. The 4GO BPM 22 exhibited the lowest Rwwithin the studied voltage range, compared to the 0GO and 1GO BPMs 22,indicating that the forward dissociation reaction is promoted moreefficiently with more added catalyst 25.

The dependence of Rw on voltage is much weaker for the 1GO/4GO BPMs,suggesting a lower electric field in BPMs 22 that contain catalystlayers 25. It is noteworthy that the different reaction resistances, Rw,between these synthetic BPMs 22 cannot be simply attributed to theco-ion leakage effect, since all BPMs 22 had similar leakage currentdensity (ses FIG. 18), but very different values of Rw. In the neutrallayer model, a reaction layer in which the water dissociation reactionbecomes prevalent and produces nearly the total amount of H⁺ and OH⁻required for a given current density is sandwiched by an EDL formed fromthe unbalanced fixed charge density on the cation exchange layer 22 aand anion exchange layer 22 b sides. This unbalanced charge is aconsequence of the depletion of ions under reverse bias conditions,resulting in the formation of a depletion region. The depletion layerthickness can be calculated from the following equation:

$d = \frac{ɛ_{0}ɛ_{r}A}{C}$

where ϵ₀ and ϵ_(r) are respectively the vacuum electric permittivity andthe dielectric constant in the reaction layer (80 was taken for purewater), and C and A are the capacitance and active membrane area (1cm²).

A depletion layer thickness, d, on the scale of hundreds of nanometersfor the 0GO BPM 22 and tens of nanometers for the 1GO and 4GO BPMs 22can be obtained. (See FIG. 21). As shown in FIG. 21, the depletionthickness d is much smaller for the 1GO and 4GO BPMs 22 than it is forthe 0GO BPM 22. The key findings from this analysis are the thinnerdepletion region and weaker dependence on electric potential withincreasing catalyst loading. This indicates that there is a smallerelectric field acting on the reaction layer in BPMs 22 that containwater dissociation catalysts, which is be evident from the results ofthe numerical modeling.

To gain further insight into the experimental data, a numerical modelwas constructed that took into account ionic transport, electrostatics,and the rates of the water dissociation/recombination reactions. Inorder to fully address the characteristics of BPMs 22, it was beneficialto identify the mechanisms that participate in the ionic transport ofboth the electrolyte and water dissociation products under bias as wellas the water dissociation/recombination reaction. A large body ofprevious work devoted to the theoretical understanding of variousphenomena in ion exchange membranes is based on theNernst-Planck-Poisson equations (NPP), where the Nernst-Planck equationdescribes ionic transport and maintains species continuity and thePoisson equation describes the fixed charge and the ion permselectivity.Incorporating the water dissociation reaction is achieved by adding aflux term in the transport equation for H⁺ and OH⁻, which also affectsthe whole system electrostatically by modifying the bulk charge density.The reaction rate can be obtained from a kinetic model for thedissociation and recombination of H⁺ and OH⁻, with a forward rateconstant that depends on the electric field and a field-independentrecombination rate constant. Two diffusion boundary layers were added atthe two faces of the BPM 22 in order to better match the experimentalconditions, and this turns out to be important in-modeling the BPM 22under higher reverse bias.

The analysis of the model begins with predictions of the J-E curve andpotential distribution profile at equilibrium. FIG. 22 shows the currentdensity at a given reverse bias and its comparison with experiment. Theagreement of the overall current density between experiment andsimulation is satisfactory at low reverse bias, whereas the deviationincreases under higher bias. This deviation could be caused by thestatic boundary conditions employed in the model, which result inunrealistic concentration profiles, as will be discussed below. Theoverall current density is decomposed into the contributions from waterdissociation products H⁺/OH⁻ and from supporting electrolyte K⁺/NO₃. Asexpected, the H⁺/OH⁻ flux surpasses that of K⁺/NO₃ only after a certainreverse bias threshold, 2V, after which the water dissociation reactionis enhanced dramatically by the electric field according to the secondWien effect. It has been shown that hysteresis develops in the J-E curveof BPMs 22 that are subjected to a time-periodic reverse voltage due tothe incomplete depletion of mobile ions at the junction, and themagnitude of the hysteresis depends on the scan rate. The absence ofhysteresis in the J-E curve, is consistent with these observations asthe current model simulates the steady-state response.

Under a reverse bias less than 5 V, more than 90% of the potential dropoccurs across the BPM junction. (See FIG. 23). At larger reverse bias of5 V, there is an appreciable potential drop in the region of electrolyteclose to the boundary. This potential drop can be attributed to the lowH⁺/OH⁻ concentration at the two boundaries, which limits the achievableH⁺/OH⁻ flux under higher reverse bias. (See FIG. 24). This also givesrise to an underestimate of the H⁺/OH⁻ concentration to the overallcurrent density. Improvement of the model may be possible by usingdynamic boundary conditions.

FIGS. 24 and 25 show the concentration profiles of H⁺/OH⁻ and K⁺/NO₃ atreverse biases of 0.3 V, 2.5 V and 5 V. These concentration profileswere found to be representative of the overall concentrationdistributions as the reverse bias varies. At lower reverse bias, thesupporting K⁺/NO₃ ions are the major charge carriers inside both the BPM22 and diffusion layers. In contrast, water dissociation products H⁺ andOH⁻ become the dominant ionic species under higher reverse bias,expelling K⁺ and NO₃ from the bulk of the membrane and accumulating inthe diffusion boundary layers. The insets in FIGS. 24 and 25 illustratethe formation of a depletion region at the cation-anion exchangejunction at an interface 23, the thickness of which increases withincreasing reverse bias. In order to assess the effectiveness of thecatalyst 25, results from the model without the catalytic effect, werecompared with the BPM 22 having a catalyst layer 25, which enhances thedissociation rate constant by two orders of magnitude. Lower currentdensity is observed at a given reverse bias compared with the BPM 22having a catalyst layer 25. As expected, the H⁺/OH⁻ flux is also smallerthan that of the BPM 22 having a catalyst layer 25 due to the lowerreaction rate constant. The onset reverse bias at which the H⁺/OH⁻ fluxstart to dominate over that of K⁺/NO₃ is lower for the BPM 22 having acatalyst layer 25, i.e. 2 V vs. 4.5 V.

The potential and concentration distribution profiles for BPMs 22 withand without a catalyst layer 25 resemble each other. Furthermore, tocheck the consistency of the model, results for the BPM 22 without acatalyst layer 25 were subjected to forward bias conditions. Currentcontributed from H⁺/OH⁻ flux is marginally small for the studied voltagerange, and the BPM 22 shows typical Ohmic resistance. The predominantcharge carriers in the BPM 22 and diffusion boundary layers are thosefrom the supporting electrolyte at all voltages. One striking differencefrom BPMs 22 under reverse bias is the absence of the depletion region,which is replaced by a smooth transition of one type of charge carrierto another. These results are in good agreement with recent theoreticalreports on BPMs in fuel cell applications where forward bias and thebackward recombination reaction are more relevant. Interestingly, mostof the potential drop under forward bias conditions happens across thediffusion layer, rather than at the cation-anion exchange junction ofthe interface 23, due to the high concentration of ions present. Havingestablished the validity of the numerical model, the electric fieldintensity was extracted at the cation-anion exchange junction \andcalculated the depletion region thickness.

FIGS. 26-28 compare the electric field intensity and depletion layerthickness for BPMs 22 with and without an interfacial catalyst layer 25.Consistent with the experimental observations, a thinner depletionregion is found for the BPM 22 having a catalyst layer 25, leading to asmaller electric field across the cation-anion exchange junction of theinterface 23. The difference can be understood as a result of thecounterbalanced roles of electric field and catalyst 25 in promotingwater dissociation. Under reverse bias, mobile ions in the BPM 22 aredriven out so that a depletion region forms due to the unbalanced fixedcharge on the anion exchange layer 22 b and cation exchange layer 22 a.The resulting electric field enhances water dissociation and producesoverwhelmingly a flux of H⁺ and OH⁻ ions towards the cation exchangelayer 22 a and anion exchange layer 22 b of the BPM 22, respectively. Assuch, the unbalanced fixed charge density is partially neutralized bythe respective counter ions, i.e. H⁺ for the cation exchange layer 22 aand OH⁻ for the anion exchange layer 22 b, hence shrinking the depletionregion. Since the H⁻/OH⁻ flux for the BPM 22 having a catalyst layer 25is much larger than BPM 22 without a catalyst layer 25, a larger portionof the fixed charge is rebalanced, causing the electric field across thereaction layer to decrease.

In some embodiments the bipolar membrane 22 can include a cationexchange layer 22 a and an anion exchange layer 22 b. The cationexchange layer 22 a can be adjacent the anion exchange layer 22 b toform a cation-anion exchange junction region of the interface 23. Insome embodiments, the cation-anion exchange junction 23 region can beconfigured as a planar junction interface. This may be referred to as atwo-dimensional junction structure or a 2D bipolar membrane 22structure. In some embodiments, the cation-anion exchange junctionregion can be configured as a network of interpenetrating anion- andcation-exchange layers 22 b, 22 a. This may be referred to as athree-dimensional junction structure or a 3D bipolar membrane 22structure. The intimate contact between the anion exchange layer 22 band cation exchange layer 22 a fibers of the 3D bipolar membrane 22 canprovide multiple transport pathways for water dissociation products H⁺and OH⁻ and greatly facilitate their removal from the cation-anionexchange junction of the interface 23. Consequently, a large H⁺/OH⁻ fluxand faster water dissociation can be achieved with an embodiment of the3D bipolar membrane 22. With embodiments of the 2D bipolar membrane 22an electric field can be applied perpendicular to the depletion layerplane. With embodiments of the 3D bipolar membrane 22, the electricfield is forced to span a range of angles relative to the membraneplane. This effect can lower the local electric field across thedispersed anion exchange layer 22 b-cation exchange layer 22 a fiberinterface 23, and thus further reduces the overall electric field. FIG.29 shows an SEM image of a cation-anion exchange junction 23 of a 3D BPM22 with intertwined anion exchange layer 22 b and cation exchange layer22 a fibers, and a schematic of the 3D BPM 22. The intimate contactbetween the anion exchange layer 22 b and cation exchange layer 22 afibers can be formed by DMF vapor treatment and hot pressing in thejunction provides multiple transport pathways for water dissociationproducts H⁺ and OH⁻ and greatly facilitates their removal from thecation-anion exchange junction 23. Consequently, a large H⁻/OH⁻ flux andfaster water dissociation are expected in the 3D cation-anion exchangejunction 23 of the BPM 22, which was experimentally confirmed. As shownabove, a large ion flux from water dissociation should compensate forthe unbalanced fixed charge in the anion exchange layer 22 b and cationexchange layer 22 a and decrease the electric field across thecation-anion exchange junction 23.

In addition, unlike in the planar junction BPM 22, in which the electricfield is applied perpendicular to the depletion layer plane, the 3Djunctions span a range of angles relative to the membrane plane, asevidenced in the SEM image of the cation-anion exchange junction 23.This effect lowers the local electric field across the dispersed anionexchange layer 22 b-cation exchange layer 22 a fiber interfaces and thusfurther reduces the overall electric field. It is the intimate localcontact between the anion exchange layer 22 b and the cation exchangelayer 22 a that distinguishes the 3D interface from its 2D counterpart.

Referring to FIGS. 30-33, under steady-state galvanostatic polarization,the 3D junction BPM 22 exhibits a similar co-ion leakage current as the4GO BPM 22 at lower reverse bias in a pH neutral electrolyte. The 3Djunction BPM 22 shows a nearly constant kd up to a reverse bias of B0.9V. It is noteworthy that the observed lower overpotential of the 3Djunction BPM 22 does not stem from catalysis 25 of the waterdissociation reaction because the 4GO BPM 22 exhibits a relativelylarger rate constant kd. In stark contrast to the 4GO BPM 22, thereaction resistance, Rw, for the 3D junction BPM 22 does not showobvious convergence to a plateau but rather remains almost constant, andis much smaller within the studied voltage. The lower reactionresistance for the 3D junction BPM 22 is attributed to facilitated waterdissociation made possible by the rapid removal of H⁺/OH⁻ through theinterpenetrating anion exchange layer 22 b and cation exchange layer 22a fibers, which results in a larger H⁺/OH⁻ flux in the cation-anionexchange junction 23 relative to flat interface BPMs 22. Theindependence of Rw on the transmembrane voltage indicates a smallelectric field in the 3D junction BPM 22 as a result of the large H⁺ andOH⁻ flux. The smaller electric field is also verified by the thinnerdepletion thickness d compared with that of the 4GO BPM 22.

Referring to FIGS. 34-34 operating parameters of embodiments of the BPMs22 under normal operating conditions were studied to gain insight intothe mechanism of water autodissociation and the effects of electricfield and catalysis. FIGS. 34-35 compare BPMs 22 with no GO/four layersof GO (0GO/4GOBPM), and the 3D junction BPM 22 with a commercialFumatech BPM in terms of the potential drop across the membrane at agiven reverse bias current density. At low current density, thecross-membrane potential is similar for all membranes except the 0GOBPM, whereas at current densities greater than 100 mA cm², and the 4GOand 3D junction BPMs 22 show significantly lower potential drop than theFumatech BPM. Galvanostatic measurements at a reverse-bias currentdensity of 100 mA cm² were performed and the results suggest that bothmembranes were stable for at least 10 hours of continuous operation. Themoderate increase in the cross-membrane potential for the 4GO BPM 22 maybe associated with degradation of GO in the interfacial layer 23 duringoperation. Compared with the BPM 22 having no catalyst layer 25, 0GO BPM22, the cross-membrane potential of the 4GO BPM 22 is much lower at allstudied reverse bias values due to the smaller water dissociationreaction resistance, Rw, of the latter. The depletion layer thicknessand thus the electric field in the 0GO BPM 22 are larger than those ofthe BPM 22 having a catalyst layer 25, 4GO BPM 22, and show a cleardependence of increasing as the reverse bias increases. For the 0GO BPM22, a wider depletion region gives rise to a stronger electric field,which promotes water dissociation to a larger extent so that theproduced H⁺/OH⁻ flux matches the higher current density at an increasedreverse bias. However, for the 4GO BPM 22, the catalyst 25 provides analternative means of enhancing the rate of water dissociation. As such,the depletion region and electric field do not need to be as enlarged inorder to achieve the same current density. Similarly, the electric fieldin the 3D junction BPM 22 is also shown to be small. Two origins for thesmall electric field in the 3D junction BPM 22 are: (1) the large H⁺/OH⁻flux due to the facile transport of the charged species because of theinterpenetrating anion exchange layer 22 b-cation exchange layer 22 adual fiber structure, compared to the incorporation of an effectivecatalyst 25 as considered in the 4GO BPM 22 and (2) the wide range ofangles spanned by the anion exchange layer 22 b-cation exchange layer 22a interfaces with respect to the overall electric field. Because of thiseffect, improving the membrane fabrication process so that the anionexchange layer 22 b-cation exchange layer 22 a interfaces are moreperpendicular to the membrane plane would be expected to impart a largerrole to the electric field in 3D junction BPMs 22.

Experiments demonstrate that BPMs 22 can be prepared from a crosslinkedanion exchange layer 22 b and Nafion cation exchange layer 22 a with aGO catalyst 25 deposited in between by layer-by-layer assemblytechnique, allowing for precise control of the interfacial 23 structure.By adjusting the GO catalyst layers 25, a balance between the secondWien effect and the catalytic effect in promoting water dissociation hasbeen discovered. A comprehensive numerical simulation model elucidatedthat the electric field enhancement for water dissociation may becompromised by incorporating catalysts into the BPM cation-anionexchange junction 23, as that produces a larger H⁺/OH⁻ flux thatpartially mitigates the net fixed charge on the anion exchange layer 22b and the cation exchange layer 22 a of the BPMs 22. This conclusion isfurther corroborated by testing a 3D junction BPM 22, which exhibits alarge H⁺/OH⁻ flux because of facilitated ionic transport through theinterpenetrating junction.

It should be understood that the disclosure of a range of values is adisclosure of every numerical value within that range, including the endpoints. It should also be appreciated that some components, features,and/or configurations may be described in connection with only oneparticular embodiment, but these same components, features, and/orconfigurations can be applied or used with many other embodiments andshould be considered applicable to the other embodiments, unless statedotherwise or unless such a component, feature, and/or configuration istechnically impossible to use with the other embodiment. Thus, thecomponents, features, and/or configurations of the various embodimentscan be combined together in any manner and such combinations areexpressly contemplated and disclosed by this statement.

It will be apparent to those skilled in the art that numerousmodifications and variations of the described examples and embodimentsare possible in light of the above teachings of the disclosure. Thedisclosed examples and embodiments are presented for purposes ofillustration only. Other alternate embodiments may include some or allof the features disclosed herein. Therefore, it is the intent to coverall such modifications and alternate embodiments as may come within thetrue scope of this invention, which is to be given the full breadththereof.

It should be understood that modifications to the embodiments disclosedherein can be made to meet a particular set of design criteria. Forinstance, any of the electrochemical cells 12, cathodes 14, anodes 18,membranes 22, catalysts 44, 46 or any other component of the device 10can be any suitable number or type of each to meet a particularobjective. Therefore, while certain exemplary embodiments of the device10 and methods of using the same disclosed herein have been discussedand illustrated, it is to be distinctly understood that the invention isnot limited thereto but may be otherwise variously embodied andpracticed within the scope of the following claims.

What is claimed is:
 1. An electrochemical device, comprising: anelectrochemical cell comprising a cathode, an anode, and a membrane;wherein: at least a portion of the cathode is separated from at least aportion of the anode by the membrane; the cathode comprises agas-diffusion electrode; the anode comprises at least one of aliquid-electrolyte style electrode and a gas-diffusion electrode; andthe membrane is a bipolar membrane, the bipolar membrane beingconfigured to maintain a flux of protons to the cathode and alsomaintain a flux of hydroxide ions to the anode, wherein theelectrochemical cell is configured to receive carbon dioxide gas andwater and output reduction products of carbon dioxide at the cathode andoxygen or other oxidized products of a depolarizer at the anode.
 2. Theelectrochemical device recited in claim 1, wherein the bipolar membranecomprises a cation exchange membrane and an anion exchange membrane. 3.The electrochemical device recited in claim 1, wherein the bipolarmembrane is configured to promote autodissociation of water.
 4. Theelectrochemical device recited in claim 1, wherein the bipolar membranefurther comprises a membrane catalyst.
 5. The electrochemical devicerecited in claim 4, wherein the membrane catalyst comprises at least oneof a silicate, an amine polymer, graphite oxide, and an anolytesolution.
 6. The electrochemical device recited in claim 2, wherein theanion exchange membrane comprises a cation-exchange polymer film.
 7. Theelectrochemical device recited in recited in claim 1, wherein theelectrochemical cell has a cell first end and a cell second end, theelectrochemical device also comprising: a cathode flow medium positionedbetween the bipolar membrane and the cathode; and an anode flow mediumpositioned between the bipolar membrane and the anode.
 8. Theelectrochemical device recited in claim 7, wherein: the cathode flowmedium has at least one cell inlet and at least one cell outlet; and theanode flow medium has at least one cell inlet and at least one celloutlet.
 9. The electrochemical device recited in claim 8, wherein thecathode flow medium comprises carbon and the anode flow medium comprisescarbon.
 10. The electrochemical device recited in claim 8, wherein: theelectrochemical device is configured as a carbon dioxide electrolyzer,the cathode comprises a cathode catalysts configured as a carbon dioxidereduction catalyst; and the anode comprises an anode catalyst configuredas a water oxidation catalyst or as a catalyst for oxidation of thedepolarizer, the depolarizer comprising hydrogen, methane, or methanol.11. The electrochemical device recited in claim 10, wherein theelectrochemical cell is configured to receive carbon dioxide gas andgenerate reduction products of carbon dioxide that include any one orcombination of formic acid, methanol, methane, formaldehyde,acetaldehyde, acetic acid, glyoxal, ethanol, ethene, ethane, ethyleneglycol, dimethyl ether, methyl formate, propene, propane, n-propanol,isopropanol, and isomers of butanol, and hydrogen.
 12. A method ofreducing product crossover in an electrochemical cell of anelectrochemical device, the method comprising: configuring a bipolarmembrane of an electrochemical cell that is positioned between an anodeand a cathode to cause ions to travel towards an anode electrode and acathode electrode of the electrochemical cell when the electrochemicalcell is under an applied current condition; operating theelectrochemical cell so that the bipolar membrane facilitates a supplyof protons (H⁺) to the cathode to cause water (H₂O) to self-ionize viaautodissociation to generate hydroxide ions (OH⁻) and protons H⁺ tosupply a flux of the OH⁻ to the anode and supply a flux of the H⁺ to thecathode.
 13. The method recited in claim 12, wherein the flux of H⁺provided by the bipolar membrane opposes product crossover in theelectrochemical cell.
 14. The method recited in claim 12, wherein thebipolar membrane has an anion exchange layer and a cation exchange layerjoined together at an interfacial layer, the interfacial layerconfigured to catalyze autodissociation of H₂O.
 15. The method recitedin claim 14, further comprising depositing at least one catalyst layeron the interfacial layer.
 16. The method recited in claim 15, furthercomprising tuning water dissociation reactions at the interfacial layervia adjusting a type of the catalyst and/or an amount of the catalyst.17. The method recited in claim 15, wherein the at least one catalystlayer comprises graphite oxide.
 18. The method recited in claim 15,wherein the cation exchange layer and the anion exchange layer define acation-anion exchange junction region; and wherein the cation-anionexchange junction is configured so that the cation exchange layerinterpenetrates the anion exchange layer and/or the anion exchange layerinterpenetrates the cation exchange layer.
 19. The method recited inclaim 18, further comprising: generating a plurality of transportpathways for water dissociation products H⁺ and OH⁻ to flow via theinterpenetrating cation exchange layer and anion exchange layer.
 20. Themethod recited in claim 19, wherein: the electrochemical device is acarbon dioxide electrolyzer, the cathode comprises a cathode catalystconfigured as a carbon dioxide reduction catalyst; the anode comprisesan anode catalyst configured as a water oxidation catalyst or as acatalyst for a depolarizer, the depolarizer comprising hydrogen,methane, or methanol; and the electrochemical cell includes: a cathodeflow medium between the cathode and the bipolar membrane, the cathodeflow medium comprising carbon, at least one cell inlet of the cathodeflow medium is configured to receive carbon dioxide, and at least onecell outlet of the cathode flow medium is configured to output carbonmonoxide gas and/or water; an anode flow medium between the anode andthe bipolar membrane, the anode flow medium comprising carbon, at leastone cell inlet of the anode flow medium configured to receive waterand/or an electrolyte and/or the depolarizer, and at least one celloutlet of the anode flow medium configured to output oxygen or theoxidized product of the depolarizer; and wherein the operating of theelectrochemical cell comprises: feeding water and/or an electrolyteand/or the depolarizer to the anode flow medium; feeding a flow ofcarbon dioxide and water to the cathode flow medium; outputting oxygenand/or the oxidation products of the depolarizer from the anode flowmedium; and outputting carbon monoxide and/or other reduction productsof carbon dioxide from the cathode flow medium.